697 lines
24 KiB
Markdown
697 lines
24 KiB
Markdown
# Docker Image Specification v1.2.0
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An *Image* is an ordered collection of root filesystem changes and the
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corresponding execution parameters for use within a container runtime. This
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specification outlines the format of these filesystem changes and corresponding
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parameters and describes how to create and use them for use with a container
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runtime and execution tool.
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This version of the image specification was adopted starting in Docker 1.12.
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## Terminology
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This specification uses the following terms:
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<dl>
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<dt>
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Layer
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</dt>
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<dd>
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Images are composed of <i>layers</i>. Each layer is a set of filesystem
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changes. Layers do not have configuration metadata such as environment
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variables or default arguments - these are properties of the image as a
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whole rather than any particular layer.
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</dd>
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<dt>
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Image JSON
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</dt>
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<dd>
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Each image has an associated JSON structure which describes some
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basic information about the image such as date created, author, and the
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ID of its parent image as well as execution/runtime configuration like
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its entry point, default arguments, CPU/memory shares, networking, and
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volumes. The JSON structure also references a cryptographic hash of
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each layer used by the image, and provides history information for
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those layers. This JSON is considered to be immutable, because changing
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it would change the computed ImageID. Changing it means creating a new
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derived image, instead of changing the existing image.
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</dd>
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<dt>
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Image Filesystem Changeset
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</dt>
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<dd>
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Each layer has an archive of the files which have been added, changed,
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or deleted relative to its parent layer. Using a layer-based or union
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filesystem such as AUFS, or by computing the diff from filesystem
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snapshots, the filesystem changeset can be used to present a series of
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image layers as if they were one cohesive filesystem.
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</dd>
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<dt>
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Layer DiffID
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</dt>
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<dd>
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Layers are referenced by cryptographic hashes of their serialized
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representation. This is a SHA256 digest over the tar archive used to
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transport the layer, represented as a hexadecimal encoding of 256 bits, e.g.,
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<code>sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
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Layers must be packed and unpacked reproducibly to avoid changing the
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layer ID, for example by using tar-split to save the tar headers. Note
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that the digest used as the layer ID is taken over an uncompressed
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version of the tar.
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</dd>
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<dt>
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Layer ChainID
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</dt>
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<dd>
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For convenience, it is sometimes useful to refer to a stack of layers
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with a single identifier. This is called a <code>ChainID</code>. For a
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single layer (or the layer at the bottom of a stack), the
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<code>ChainID</code> is equal to the layer's <code>DiffID</code>.
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Otherwise the <code>ChainID</code> is given by the formula:
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<code>ChainID(layerN) = SHA256hex(ChainID(layerN-1) + " " + DiffID(layerN))</code>.
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</dd>
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<dt>
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ImageID <a name="id_desc"></a>
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</dt>
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<dd>
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Each image's ID is given by the SHA256 hash of its configuration JSON. It is
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represented as a hexadecimal encoding of 256 bits, e.g.,
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<code>sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
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Since the configuration JSON that gets hashed references hashes of each
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layer in the image, this formulation of the ImageID makes images
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content-addresable.
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</dd>
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<dt>
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Tag
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</dt>
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<dd>
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A tag serves to map a descriptive, user-given name to any single image
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ID. Tag values are limited to the set of characters
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<code>[a-zA-Z0-9_.-]</code>, except they may not start with a <code>.</code>
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or <code>-</code> character. Tags are limited to 127 characters.
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</dd>
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<dt>
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Repository
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</dt>
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<dd>
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A collection of tags grouped under a common prefix (the name component
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before <code>:</code>). For example, in an image tagged with the name
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<code>my-app:3.1.4</code>, <code>my-app</code> is the <i>Repository</i>
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component of the name. A repository name is made up of slash-separated
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name components, optionally prefixed by a DNS hostname. The hostname
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must follow comply with standard DNS rules, but may not contain
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<code>_</code> characters. If a hostname is present, it may optionally
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be followed by a port number in the format <code>:8080</code>.
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Name components may contain lowercase characters, digits, and
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separators. A separator is defined as a period, one or two underscores,
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or one or more dashes. A name component may not start or end with
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a separator.
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</dd>
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</dl>
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## Image JSON Description
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Here is an example image JSON file:
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```
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{
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"created": "2015-10-31T22:22:56.015925234Z",
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"author": "Alyssa P. Hacker <alyspdev@example.com>",
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"architecture": "amd64",
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"os": "linux",
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"config": {
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"User": "alice",
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"Memory": 2048,
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"MemorySwap": 4096,
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"CpuShares": 8,
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"ExposedPorts": {
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"8080/tcp": {}
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},
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"Env": [
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"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
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"FOO=docker_is_a_really",
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"BAR=great_tool_you_know"
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],
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"Entrypoint": [
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"/bin/my-app-binary"
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],
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"Cmd": [
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"--foreground",
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"--config",
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"/etc/my-app.d/default.cfg"
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],
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"Volumes": {
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"/var/job-result-data": {},
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"/var/log/my-app-logs": {},
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},
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"WorkingDir": "/home/alice",
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},
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"rootfs": {
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"diff_ids": [
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"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
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"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
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],
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"type": "layers"
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},
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"history": [
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{
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"created": "2015-10-31T22:22:54.690851953Z",
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"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
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},
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{
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"created": "2015-10-31T22:22:55.613815829Z",
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"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
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"empty_layer": true
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}
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]
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}
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```
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Note that image JSON files produced by Docker don't contain formatting
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whitespace. It has been added to this example for clarity.
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### Image JSON Field Descriptions
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<dl>
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<dt>
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created <code>string</code>
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</dt>
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<dd>
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ISO-8601 formatted combined date and time at which the image was
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created.
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</dd>
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<dt>
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author <code>string</code>
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</dt>
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<dd>
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Gives the name and/or email address of the person or entity which
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created and is responsible for maintaining the image.
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</dd>
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<dt>
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architecture <code>string</code>
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</dt>
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<dd>
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The CPU architecture which the binaries in this image are built to run
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on. Possible values include:
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<ul>
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<li>386</li>
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<li>amd64</li>
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<li>arm</li>
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</ul>
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More values may be supported in the future and any of these may or may
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not be supported by a given container runtime implementation.
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</dd>
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<dt>
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os <code>string</code>
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</dt>
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<dd>
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The name of the operating system which the image is built to run on.
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Possible values include:
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<ul>
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<li>darwin</li>
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<li>freebsd</li>
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<li>linux</li>
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</ul>
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More values may be supported in the future and any of these may or may
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not be supported by a given container runtime implementation.
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</dd>
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<dt>
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config <code>struct</code>
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</dt>
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<dd>
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The execution parameters which should be used as a base when running a
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container using the image. This field can be <code>null</code>, in
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which case any execution parameters should be specified at creation of
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the container.
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<h4>Container RunConfig Field Descriptions</h4>
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<dl>
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<dt>
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User <code>string</code>
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</dt>
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<dd>
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<p>The username or UID which the process in the container should
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run as. This acts as a default value to use when the value is
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not specified when creating a container.</p>
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<p>All of the following are valid:</p>
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<ul>
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<li><code>user</code></li>
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<li><code>uid</code></li>
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<li><code>user:group</code></li>
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<li><code>uid:gid</code></li>
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<li><code>uid:group</code></li>
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<li><code>user:gid</code></li>
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</ul>
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<p>If <code>group</code>/<code>gid</code> is not specified, the
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default group and supplementary groups of the given
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<code>user</code>/<code>uid</code> in <code>/etc/passwd</code>
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from the container are applied.</p>
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</dd>
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<dt>
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Memory <code>integer</code>
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</dt>
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<dd>
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Memory limit (in bytes). This acts as a default value to use
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when the value is not specified when creating a container.
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</dd>
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<dt>
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MemorySwap <code>integer</code>
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</dt>
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<dd>
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Total memory usage (memory + swap); set to <code>-1</code> to
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disable swap. This acts as a default value to use when the
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value is not specified when creating a container.
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</dd>
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<dt>
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CpuShares <code>integer</code>
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</dt>
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<dd>
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CPU shares (relative weight vs. other containers). This acts as
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a default value to use when the value is not specified when
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creating a container.
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</dd>
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<dt>
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ExposedPorts <code>struct</code>
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</dt>
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<dd>
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A set of ports to expose from a container running this image.
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This JSON structure value is unusual because it is a direct
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JSON serialization of the Go type
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<code>map[string]struct{}</code> and is represented in JSON as
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an object mapping its keys to an empty object. Here is an
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example:
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<pre>{
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"8080": {},
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"53/udp": {},
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"2356/tcp": {}
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}</pre>
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Its keys can be in the format of:
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<ul>
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<li>
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<code>"port/tcp"</code>
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</li>
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<li>
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<code>"port/udp"</code>
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</li>
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<li>
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<code>"port"</code>
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</li>
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</ul>
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with the default protocol being <code>"tcp"</code> if not
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specified.
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These values act as defaults and are merged with any specified
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when creating a container.
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</dd>
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<dt>
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Env <code>array of strings</code>
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</dt>
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<dd>
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Entries are in the format of <code>VARNAME="var value"</code>.
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These values act as defaults and are merged with any specified
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when creating a container.
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</dd>
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<dt>
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Entrypoint <code>array of strings</code>
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</dt>
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<dd>
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A list of arguments to use as the command to execute when the
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container starts. This value acts as a default and is replaced
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by an entrypoint specified when creating a container.
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</dd>
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<dt>
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Cmd <code>array of strings</code>
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</dt>
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<dd>
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Default arguments to the entry point of the container. These
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values act as defaults and are replaced with any specified when
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creating a container. If an <code>Entrypoint</code> value is
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not specified, then the first entry of the <code>Cmd</code>
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array should be interpreted as the executable to run.
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</dd>
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<dt>
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Healthcheck <code>struct</code>
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</dt>
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<dd>
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A test to perform to determine whether the container is healthy.
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Here is an example:
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<pre>{
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"Test": [
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"CMD-SHELL",
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"/usr/bin/check-health localhost"
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],
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"Interval": 30000000000,
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"Timeout": 10000000000,
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"Retries": 3
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}</pre>
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The object has the following fields.
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<dl>
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<dt>
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Test <code>array of strings</code>
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</dt>
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<dd>
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The test to perform to check that the container is healthy.
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The options are:
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<ul>
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<li><code>[]</code> : inherit healthcheck from base image</li>
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<li><code>["NONE"]</code> : disable healthcheck</li>
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<li><code>["CMD", arg1, arg2, ...]</code> : exec arguments directly</li>
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<li><code>["CMD-SHELL", command]</code> : run command with system's default shell</li>
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</ul>
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The test command should exit with a status of 0 if the container is healthy,
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or with 1 if it is unhealthy.
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</dd>
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<dt>
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Interval <code>integer</code>
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</dt>
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<dd>
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Number of nanoseconds to wait between probe attempts.
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</dd>
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<dt>
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Timeout <code>integer</code>
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</dt>
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<dd>
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Number of nanoseconds to wait before considering the check to have hung.
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</dd>
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<dt>
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Retries <code>integer</code>
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<dt>
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<dd>
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The number of consecutive failures needed to consider a container as unhealthy.
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</dd>
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</dl>
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In each case, the field can be omitted to indicate that the
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value should be inherited from the base layer.
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These values act as defaults and are merged with any specified
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when creating a container.
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</dd>
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<dt>
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Volumes <code>struct</code>
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</dt>
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<dd>
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A set of directories which should be created as data volumes in
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a container running this image. This JSON structure value is
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unusual because it is a direct JSON serialization of the Go
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type <code>map[string]struct{}</code> and is represented in
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JSON as an object mapping its keys to an empty object. Here is
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an example:
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<pre>{
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"/var/my-app-data/": {},
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"/etc/some-config.d/": {},
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}</pre>
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</dd>
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<dt>
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WorkingDir <code>string</code>
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</dt>
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<dd>
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Sets the current working directory of the entry point process
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in the container. This value acts as a default and is replaced
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by a working directory specified when creating a container.
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</dd>
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</dl>
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</dd>
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<dt>
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rootfs <code>struct</code>
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</dt>
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<dd>
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The rootfs key references the layer content addresses used by the
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image. This makes the image config hash depend on the filesystem hash.
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rootfs has two subkeys:
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<ul>
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<li>
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<code>type</code> is usually set to <code>layers</code>.
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</li>
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<li>
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<code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most.
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</li>
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</ul>
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Here is an example rootfs section:
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<pre>"rootfs": {
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"diff_ids": [
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"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
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"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef",
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"sha256:13f53e08df5a220ab6d13c58b2bf83a59cbdc2e04d0a3f041ddf4b0ba4112d49"
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],
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"type": "layers"
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}</pre>
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</dd>
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<dt>
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history <code>struct</code>
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</dt>
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<dd>
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<code>history</code> is an array of objects describing the history of
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each layer. The array is ordered from bottom-most layer to top-most
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layer. The object has the following fields.
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<ul>
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<li>
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<code>created</code>: Creation time, expressed as a ISO-8601 formatted
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combined date and time
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</li>
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<li>
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<code>author</code>: The author of the build point
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</li>
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<li>
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<code>created_by</code>: The command which created the layer
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</li>
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<li>
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<code>comment</code>: A custom message set when creating the layer
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</li>
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<li>
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<code>empty_layer</code>: This field is used to mark if the history
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item created a filesystem diff. It is set to true if this history
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item doesn't correspond to an actual layer in the rootfs section
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(for example, a command like ENV which results in no change to the
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filesystem).
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</li>
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</ul>
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Here is an example history section:
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<pre>"history": [
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{
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"created": "2015-10-31T22:22:54.690851953Z",
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"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
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},
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{
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"created": "2015-10-31T22:22:55.613815829Z",
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"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
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"empty_layer": true
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}
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]</pre>
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</dd>
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</dl>
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Any extra fields in the Image JSON struct are considered implementation
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specific and should be ignored by any implementations which are unable to
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interpret them.
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## Creating an Image Filesystem Changeset
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An example of creating an Image Filesystem Changeset follows.
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An image root filesystem is first created as an empty directory. Here is the
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initial empty directory structure for the a changeset using the
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randomly-generated directory name `c3167915dc9d` ([actual layer DiffIDs are
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generated based on the content](#id_desc)).
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```
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c3167915dc9d/
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```
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|
|
|
Files and directories are then created:
|
|
|
|
```
|
|
c3167915dc9d/
|
|
etc/
|
|
my-app-config
|
|
bin/
|
|
my-app-binary
|
|
my-app-tools
|
|
```
|
|
|
|
The `c3167915dc9d` directory is then committed as a plain Tar archive with
|
|
entries for the following files:
|
|
|
|
```
|
|
etc/my-app-config
|
|
bin/my-app-binary
|
|
bin/my-app-tools
|
|
```
|
|
|
|
To make changes to the filesystem of this container image, create a new
|
|
directory, such as `f60c56784b83`, and initialize it with a snapshot of the
|
|
parent image's root filesystem, so that the directory is identical to that
|
|
of `c3167915dc9d`. NOTE: a copy-on-write or union filesystem can make this very
|
|
efficient:
|
|
|
|
```
|
|
f60c56784b83/
|
|
etc/
|
|
my-app-config
|
|
bin/
|
|
my-app-binary
|
|
my-app-tools
|
|
```
|
|
|
|
This example change is going add a configuration directory at `/etc/my-app.d`
|
|
which contains a default config file. There's also a change to the
|
|
`my-app-tools` binary to handle the config layout change. The `f60c56784b83`
|
|
directory then looks like this:
|
|
|
|
```
|
|
f60c56784b83/
|
|
etc/
|
|
my-app.d/
|
|
default.cfg
|
|
bin/
|
|
my-app-binary
|
|
my-app-tools
|
|
```
|
|
|
|
This reflects the removal of `/etc/my-app-config` and creation of a file and
|
|
directory at `/etc/my-app.d/default.cfg`. `/bin/my-app-tools` has also been
|
|
replaced with an updated version. Before committing this directory to a
|
|
changeset, because it has a parent image, it is first compared with the
|
|
directory tree of the parent snapshot, `f60c56784b83`, looking for files and
|
|
directories that have been added, modified, or removed. The following changeset
|
|
is found:
|
|
|
|
```
|
|
Added: /etc/my-app.d/default.cfg
|
|
Modified: /bin/my-app-tools
|
|
Deleted: /etc/my-app-config
|
|
```
|
|
|
|
A Tar Archive is then created which contains *only* this changeset: The added
|
|
and modified files and directories in their entirety, and for each deleted item
|
|
an entry for an empty file at the same location but with the basename of the
|
|
deleted file or directory prefixed with `.wh.`. The filenames prefixed with
|
|
`.wh.` are known as "whiteout" files. NOTE: For this reason, it is not possible
|
|
to create an image root filesystem which contains a file or directory with a
|
|
name beginning with `.wh.`. The resulting Tar archive for `f60c56784b83` has
|
|
the following entries:
|
|
|
|
```
|
|
/etc/my-app.d/default.cfg
|
|
/bin/my-app-tools
|
|
/etc/.wh.my-app-config
|
|
```
|
|
|
|
Any given image is likely to be composed of several of these Image Filesystem
|
|
Changeset tar archives.
|
|
|
|
## Combined Image JSON + Filesystem Changeset Format
|
|
|
|
There is also a format for a single archive which contains complete information
|
|
about an image, including:
|
|
|
|
- repository names/tags
|
|
- image configuration JSON file
|
|
- all tar archives of each layer filesystem changesets
|
|
|
|
For example, here's what the full archive of `library/busybox` is (displayed in
|
|
`tree` format):
|
|
|
|
```
|
|
.
|
|
├── 47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json
|
|
├── 5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a
|
|
│ ├── VERSION
|
|
│ ├── json
|
|
│ └── layer.tar
|
|
├── a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198
|
|
│ ├── VERSION
|
|
│ ├── json
|
|
│ └── layer.tar
|
|
├── manifest.json
|
|
└── repositories
|
|
```
|
|
|
|
There is a directory for each layer in the image. Each directory is named with
|
|
a 64 character hex name that is deterministically generated from the layer
|
|
information. These names are not necessarily layer DiffIDs or ChainIDs. Each of
|
|
these directories contains 3 files:
|
|
|
|
* `VERSION` - The schema version of the `json` file
|
|
* `json` - The legacy JSON metadata for an image layer. In this version of
|
|
the image specification, layers don't have JSON metadata, but in
|
|
[version 1](v1.md), they did. A file is created for each layer in the
|
|
v1 format for backward compatibility.
|
|
* `layer.tar` - The Tar archive of the filesystem changeset for an image
|
|
layer.
|
|
|
|
Note that this directory layout is only important for backward compatibility.
|
|
Current implementations use the paths specified in `manifest.json`.
|
|
|
|
The content of the `VERSION` files is simply the semantic version of the JSON
|
|
metadata schema:
|
|
|
|
```
|
|
1.0
|
|
```
|
|
|
|
The `repositories` file is another JSON file which describes names/tags:
|
|
|
|
```
|
|
{
|
|
"busybox":{
|
|
"latest":"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a"
|
|
}
|
|
}
|
|
```
|
|
|
|
Every key in this object is the name of a repository, and maps to a collection
|
|
of tag suffixes. Each tag maps to the ID of the image represented by that tag.
|
|
This file is only used for backwards compatibility. Current implementations use
|
|
the `manifest.json` file instead.
|
|
|
|
The `manifest.json` file provides the image JSON for the top-level image, and
|
|
optionally for parent images that this image was derived from. It consists of
|
|
an array of metadata entries:
|
|
|
|
```
|
|
[
|
|
{
|
|
"Config": "47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json",
|
|
"RepoTags": ["busybox:latest"],
|
|
"Layers": [
|
|
"a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198/layer.tar",
|
|
"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a/layer.tar"
|
|
]
|
|
}
|
|
]
|
|
```
|
|
|
|
There is an entry in the array for each image.
|
|
|
|
The `Config` field references another file in the tar which includes the image
|
|
JSON for this image.
|
|
|
|
The `RepoTags` field lists references pointing to this image.
|
|
|
|
The `Layers` field points to the filesystem changeset tars.
|
|
|
|
An optional `Parent` field references the imageID of the parent image. This
|
|
parent must be part of the same `manifest.json` file.
|
|
|
|
This file shouldn't be confused with the distribution manifest, used to push
|
|
and pull images.
|
|
|
|
Generally, implementations that support this version of the spec will use
|
|
the `manifest.json` file if available, and older implementations will use the
|
|
legacy `*/json` files and `repositories`.
|