mirror of
https://github.com/moby/moby.git
synced 2022-11-09 12:21:53 -05:00
4fb59c20a4
Embedded new lines aren't interpreted correctly in markdown renderers (they are treated as preformatted text instead). I removed the embedded newlines in the docker image spec. Signed-off-by: Alex Goodman <wagoodman@gmail.com>
623 lines
22 KiB
Markdown
623 lines
22 KiB
Markdown
# Docker Image Specification v1.1.0
|
|
|
|
An *Image* is an ordered collection of root filesystem changes and the
|
|
corresponding execution parameters for use within a container runtime. This
|
|
specification outlines the format of these filesystem changes and corresponding
|
|
parameters and describes how to create and use them for use with a container
|
|
runtime and execution tool.
|
|
|
|
This version of the image specification was adopted starting in Docker 1.10.
|
|
|
|
## Terminology
|
|
|
|
This specification uses the following terms:
|
|
|
|
<dl>
|
|
<dt>
|
|
Layer
|
|
</dt>
|
|
<dd>
|
|
Images are composed of <i>layers</i>. Each layer is a set of filesystem
|
|
changes. Layers do not have configuration metadata such as environment
|
|
variables or default arguments - these are properties of the image as a
|
|
whole rather than any particular layer.
|
|
</dd>
|
|
<dt>
|
|
Image JSON
|
|
</dt>
|
|
<dd>
|
|
Each image has an associated JSON structure which describes some
|
|
basic information about the image such as date created, author, and the
|
|
ID of its parent image as well as execution/runtime configuration like
|
|
its entry point, default arguments, CPU/memory shares, networking, and
|
|
volumes. The JSON structure also references a cryptographic hash of
|
|
each layer used by the image, and provides history information for
|
|
those layers. This JSON is considered to be immutable, because changing
|
|
it would change the computed ImageID. Changing it means creating a new
|
|
derived image, instead of changing the existing image.
|
|
</dd>
|
|
<dt>
|
|
Image Filesystem Changeset
|
|
</dt>
|
|
<dd>
|
|
Each layer has an archive of the files which have been added, changed,
|
|
or deleted relative to its parent layer. Using a layer-based or union
|
|
filesystem such as AUFS, or by computing the diff from filesystem
|
|
snapshots, the filesystem changeset can be used to present a series of
|
|
image layers as if they were one cohesive filesystem.
|
|
</dd>
|
|
<dt>
|
|
Layer DiffID
|
|
</dt>
|
|
<dd>
|
|
Layers are referenced by cryptographic hashes of their serialized
|
|
representation. This is a SHA256 digest over the tar archive used to
|
|
transport the layer, represented as a hexadecimal encoding of 256 bits, e.g.,
|
|
<code>sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
|
|
Layers must be packed and unpacked reproducibly to avoid changing the
|
|
layer ID, for example by using tar-split to save the tar headers. Note
|
|
that the digest used as the layer ID is taken over an uncompressed
|
|
version of the tar.
|
|
</dd>
|
|
<dt>
|
|
Layer ChainID
|
|
</dt>
|
|
<dd>
|
|
For convenience, it is sometimes useful to refer to a stack of layers
|
|
with a single identifier. This is called a <code>ChainID</code>. For a
|
|
single layer (or the layer at the bottom of a stack), the
|
|
<code>ChainID</code> is equal to the layer's <code>DiffID</code>.
|
|
Otherwise the <code>ChainID</code> is given by the formula:
|
|
<code>ChainID(layerN) = SHA256hex(ChainID(layerN-1) + " " + DiffID(layerN))</code>.
|
|
</dd>
|
|
<dt>
|
|
ImageID <a name="id_desc"></a>
|
|
</dt>
|
|
<dd>
|
|
Each image's ID is given by the SHA256 hash of its configuration JSON. It is
|
|
represented as a hexadecimal encoding of 256 bits, e.g.,
|
|
<code>sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
|
|
Since the configuration JSON that gets hashed references hashes of each
|
|
layer in the image, this formulation of the ImageID makes images
|
|
content-addressable.
|
|
</dd>
|
|
<dt>
|
|
Tag
|
|
</dt>
|
|
<dd>
|
|
A tag serves to map a descriptive, user-given name to any single image
|
|
ID. Tag values are limited to the set of characters
|
|
<code>[a-zA-Z0-9_.-]</code>, except they may not start with a <code>.</code>
|
|
or <code>-</code> character. Tags are limited to 128 characters.
|
|
</dd>
|
|
<dt>
|
|
Repository
|
|
</dt>
|
|
<dd>
|
|
A collection of tags grouped under a common prefix (the name component
|
|
before <code>:</code>). For example, in an image tagged with the name
|
|
<code>my-app:3.1.4</code>, <code>my-app</code> is the <i>Repository</i>
|
|
component of the name. A repository name is made up of slash-separated
|
|
name components, optionally prefixed by a DNS hostname. The hostname
|
|
must comply with standard DNS rules, but may not contain
|
|
<code>_</code> characters. If a hostname is present, it may optionally
|
|
be followed by a port number in the format <code>:8080</code>.
|
|
Name components may contain lowercase characters, digits, and
|
|
separators. A separator is defined as a period, one or two underscores,
|
|
or one or more dashes. A name component may not start or end with
|
|
a separator.
|
|
</dd>
|
|
</dl>
|
|
|
|
## Image JSON Description
|
|
|
|
Here is an example image JSON file:
|
|
|
|
```
|
|
{
|
|
"created": "2015-10-31T22:22:56.015925234Z",
|
|
"author": "Alyssa P. Hacker <alyspdev@example.com>",
|
|
"architecture": "amd64",
|
|
"os": "linux",
|
|
"config": {
|
|
"User": "alice",
|
|
"Memory": 2048,
|
|
"MemorySwap": 4096,
|
|
"CpuShares": 8,
|
|
"ExposedPorts": {
|
|
"8080/tcp": {}
|
|
},
|
|
"Env": [
|
|
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
|
|
"FOO=docker_is_a_really",
|
|
"BAR=great_tool_you_know"
|
|
],
|
|
"Entrypoint": [
|
|
"/bin/my-app-binary"
|
|
],
|
|
"Cmd": [
|
|
"--foreground",
|
|
"--config",
|
|
"/etc/my-app.d/default.cfg"
|
|
],
|
|
"Volumes": {
|
|
"/var/job-result-data": {},
|
|
"/var/log/my-app-logs": {},
|
|
},
|
|
"WorkingDir": "/home/alice",
|
|
},
|
|
"rootfs": {
|
|
"diff_ids": [
|
|
"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
|
|
"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
|
|
],
|
|
"type": "layers"
|
|
},
|
|
"history": [
|
|
{
|
|
"created": "2015-10-31T22:22:54.690851953Z",
|
|
"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
|
|
},
|
|
{
|
|
"created": "2015-10-31T22:22:55.613815829Z",
|
|
"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
|
|
"empty_layer": true
|
|
}
|
|
]
|
|
}
|
|
```
|
|
|
|
Note that image JSON files produced by Docker don't contain formatting
|
|
whitespace. It has been added to this example for clarity.
|
|
|
|
### Image JSON Field Descriptions
|
|
|
|
<dl>
|
|
<dt>
|
|
created <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
ISO-8601 formatted combined date and time at which the image was
|
|
created.
|
|
</dd>
|
|
<dt>
|
|
author <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
Gives the name and/or email address of the person or entity which
|
|
created and is responsible for maintaining the image.
|
|
</dd>
|
|
<dt>
|
|
architecture <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
The CPU architecture which the binaries in this image are built to run
|
|
on. Possible values include:
|
|
<ul>
|
|
<li>386</li>
|
|
<li>amd64</li>
|
|
<li>arm</li>
|
|
</ul>
|
|
More values may be supported in the future and any of these may or may
|
|
not be supported by a given container runtime implementation.
|
|
</dd>
|
|
<dt>
|
|
os <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
The name of the operating system which the image is built to run on.
|
|
Possible values include:
|
|
<ul>
|
|
<li>darwin</li>
|
|
<li>freebsd</li>
|
|
<li>linux</li>
|
|
</ul>
|
|
More values may be supported in the future and any of these may or may
|
|
not be supported by a given container runtime implementation.
|
|
</dd>
|
|
<dt>
|
|
config <code>struct</code>
|
|
</dt>
|
|
<dd>
|
|
The execution parameters which should be used as a base when running a
|
|
container using the image. This field can be <code>null</code>, in
|
|
which case any execution parameters should be specified at creation of
|
|
the container.
|
|
<h4>Container RunConfig Field Descriptions</h4>
|
|
<dl>
|
|
<dt>
|
|
User <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
<p>The username or UID which the process in the container should
|
|
run as. This acts as a default value to use when the value is
|
|
not specified when creating a container.</p>
|
|
<p>All of the following are valid:</p>
|
|
<ul>
|
|
<li><code>user</code></li>
|
|
<li><code>uid</code></li>
|
|
<li><code>user:group</code></li>
|
|
<li><code>uid:gid</code></li>
|
|
<li><code>uid:group</code></li>
|
|
<li><code>user:gid</code></li>
|
|
</ul>
|
|
<p>If <code>group</code>/<code>gid</code> is not specified, the
|
|
default group and supplementary groups of the given
|
|
<code>user</code>/<code>uid</code> in <code>/etc/passwd</code>
|
|
from the container are applied.</p>
|
|
</dd>
|
|
<dt>
|
|
Memory <code>integer</code>
|
|
</dt>
|
|
<dd>
|
|
Memory limit (in bytes). This acts as a default value to use
|
|
when the value is not specified when creating a container.
|
|
</dd>
|
|
<dt>
|
|
MemorySwap <code>integer</code>
|
|
</dt>
|
|
<dd>
|
|
Total memory usage (memory + swap); set to <code>-1</code> to
|
|
disable swap. This acts as a default value to use when the
|
|
value is not specified when creating a container.
|
|
</dd>
|
|
<dt>
|
|
CpuShares <code>integer</code>
|
|
</dt>
|
|
<dd>
|
|
CPU shares (relative weight vs. other containers). This acts as
|
|
a default value to use when the value is not specified when
|
|
creating a container.
|
|
</dd>
|
|
<dt>
|
|
ExposedPorts <code>struct</code>
|
|
</dt>
|
|
<dd>
|
|
A set of ports to expose from a container running this image.
|
|
This JSON structure value is unusual because it is a direct
|
|
JSON serialization of the Go type
|
|
<code>map[string]struct{}</code> and is represented in JSON as
|
|
an object mapping its keys to an empty object. Here is an
|
|
example:
|
|
<pre>{
|
|
"8080": {},
|
|
"53/udp": {},
|
|
"2356/tcp": {}
|
|
}</pre>
|
|
Its keys can be in the format of:
|
|
<ul>
|
|
<li>
|
|
<code>"port/tcp"</code>
|
|
</li>
|
|
<li>
|
|
<code>"port/udp"</code>
|
|
</li>
|
|
<li>
|
|
<code>"port"</code>
|
|
</li>
|
|
</ul>
|
|
with the default protocol being <code>"tcp"</code> if not
|
|
specified. These values act as defaults and are merged with any
|
|
specified when creating a container.
|
|
</dd>
|
|
<dt>
|
|
Env <code>array of strings</code>
|
|
</dt>
|
|
<dd>
|
|
Entries are in the format of <code>VARNAME="var value"</code>.
|
|
These values act as defaults and are merged with any specified
|
|
when creating a container.
|
|
</dd>
|
|
<dt>
|
|
Entrypoint <code>array of strings</code>
|
|
</dt>
|
|
<dd>
|
|
A list of arguments to use as the command to execute when the
|
|
container starts. This value acts as a default and is replaced
|
|
by an entrypoint specified when creating a container.
|
|
</dd>
|
|
<dt>
|
|
Cmd <code>array of strings</code>
|
|
</dt>
|
|
<dd>
|
|
Default arguments to the entry point of the container. These
|
|
values act as defaults and are replaced with any specified when
|
|
creating a container. If an <code>Entrypoint</code> value is
|
|
not specified, then the first entry of the <code>Cmd</code>
|
|
array should be interpreted as the executable to run.
|
|
</dd>
|
|
<dt>
|
|
Volumes <code>struct</code>
|
|
</dt>
|
|
<dd>
|
|
A set of directories which should be created as data volumes in
|
|
a container running this image. This JSON structure value is
|
|
unusual because it is a direct JSON serialization of the Go
|
|
type <code>map[string]struct{}</code> and is represented in
|
|
JSON as an object mapping its keys to an empty object. Here is
|
|
an example:
|
|
<pre>{
|
|
"/var/my-app-data/": {},
|
|
"/etc/some-config.d/": {},
|
|
}</pre>
|
|
</dd>
|
|
<dt>
|
|
WorkingDir <code>string</code>
|
|
</dt>
|
|
<dd>
|
|
Sets the current working directory of the entry point process
|
|
in the container. This value acts as a default and is replaced
|
|
by a working directory specified when creating a container.
|
|
</dd>
|
|
</dl>
|
|
</dd>
|
|
<dt>
|
|
rootfs <code>struct</code>
|
|
</dt>
|
|
<dd>
|
|
The rootfs key references the layer content addresses used by the
|
|
image. This makes the image config hash depend on the filesystem hash.
|
|
rootfs has two subkeys:
|
|
<ul>
|
|
<li>
|
|
<code>type</code> is usually set to <code>layers</code>.
|
|
</li>
|
|
<li>
|
|
<code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most.
|
|
</li>
|
|
</ul>
|
|
Here is an example rootfs section:
|
|
<pre>"rootfs": {
|
|
"diff_ids": [
|
|
"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
|
|
"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef",
|
|
"sha256:13f53e08df5a220ab6d13c58b2bf83a59cbdc2e04d0a3f041ddf4b0ba4112d49"
|
|
],
|
|
"type": "layers"
|
|
}</pre>
|
|
</dd>
|
|
<dt>
|
|
history <code>struct</code>
|
|
</dt>
|
|
<dd>
|
|
<code>history</code> is an array of objects describing the history of
|
|
each layer. The array is ordered from bottom-most layer to top-most
|
|
layer. The object has the following fields.
|
|
<ul>
|
|
<li>
|
|
<code>created</code>: Creation time, expressed as a ISO-8601 formatted
|
|
combined date and time
|
|
</li>
|
|
<li>
|
|
<code>author</code>: The author of the build point
|
|
</li>
|
|
<li>
|
|
<code>created_by</code>: The command which created the layer
|
|
</li>
|
|
<li>
|
|
<code>comment</code>: A custom message set when creating the layer
|
|
</li>
|
|
<li>
|
|
<code>empty_layer</code>: This field is used to mark if the history
|
|
item created a filesystem diff. It is set to true if this history
|
|
item doesn't correspond to an actual layer in the rootfs section
|
|
(for example, a command like ENV which results in no change to the
|
|
filesystem).
|
|
</li>
|
|
</ul>
|
|
|
|
Here is an example history section:
|
|
|
|
<pre>"history": [
|
|
{
|
|
"created": "2015-10-31T22:22:54.690851953Z",
|
|
"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
|
|
},
|
|
{
|
|
"created": "2015-10-31T22:22:55.613815829Z",
|
|
"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
|
|
"empty_layer": true
|
|
}
|
|
]</pre>
|
|
</dd>
|
|
</dl>
|
|
|
|
Any extra fields in the Image JSON struct are considered implementation
|
|
specific and should be ignored by any implementations which are unable to
|
|
interpret them.
|
|
|
|
## Creating an Image Filesystem Changeset
|
|
|
|
An example of creating an Image Filesystem Changeset follows.
|
|
|
|
An image root filesystem is first created as an empty directory. Here is the
|
|
initial empty directory structure for the a changeset using the
|
|
randomly-generated directory name `c3167915dc9d` ([actual layer DiffIDs are
|
|
generated based on the content](#id_desc)).
|
|
|
|
```
|
|
c3167915dc9d/
|
|
```
|
|
|
|
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`.
|