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Get started with images
In the introduction we've discovered that Docker
images are the basis of containers. In the
previous sections
we've used Docker images that already exist, for example the ubuntu
image and the training/webapp
image.
We've also discovered that Docker stores downloaded images on the Docker host. If an image isn't already present on the host then it'll be downloaded from a registry: by default the Docker Hub Registry.
In this section we're going to explore Docker images a bit more including:
- Managing and working with images locally on your Docker host;
- Creating basic images;
- Uploading images to Docker Hub Registry.
Listing images on the host
Let's start with listing the images we have locally on our host. You can
do this using the docker images
command like so:
$ docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
training/webapp latest fc77f57ad303 3 weeks ago 280.5 MB
ubuntu 13.10 5e019ab7bf6d 4 weeks ago 180 MB
ubuntu saucy 5e019ab7bf6d 4 weeks ago 180 MB
ubuntu 12.04 74fe38d11401 4 weeks ago 209.6 MB
ubuntu precise 74fe38d11401 4 weeks ago 209.6 MB
ubuntu 12.10 a7cf8ae4e998 4 weeks ago 171.3 MB
ubuntu quantal a7cf8ae4e998 4 weeks ago 171.3 MB
ubuntu 14.04 99ec81b80c55 4 weeks ago 266 MB
ubuntu latest 99ec81b80c55 4 weeks ago 266 MB
ubuntu trusty 99ec81b80c55 4 weeks ago 266 MB
ubuntu 13.04 316b678ddf48 4 weeks ago 169.4 MB
ubuntu raring 316b678ddf48 4 weeks ago 169.4 MB
ubuntu 10.04 3db9c44f4520 4 weeks ago 183 MB
ubuntu lucid 3db9c44f4520 4 weeks ago 183 MB
We can see the images we've previously used in our user guide. Each has been downloaded from Docker Hub when we launched a container using that image.
We can see three crucial pieces of information about our images in the listing.
- What repository they came from, for example
ubuntu
. - The tags for each image, for example
14.04
. - The image ID of each image.
Note: Previously, the
docker images
command supported the--tree
and--dot
arguments, which displayed different visualizations of the image data. Docker core removed this functionality in the 1.7 version. If you liked this functionality, you can still find it in the third-party dockviz tool.
A repository potentially holds multiple variants of an image. In the case of
our ubuntu
image we can see multiple variants covering Ubuntu 10.04, 12.04,
12.10, 13.04, 13.10 and 14.04. Each variant is identified by a tag and you can
refer to a tagged image like so:
ubuntu:14.04
So when we run a container we refer to a tagged image like so:
$ docker run -t -i ubuntu:14.04 /bin/bash
If instead we wanted to run an Ubuntu 12.04 image we'd use:
$ docker run -t -i ubuntu:12.04 /bin/bash
If you don't specify a variant, for example you just use ubuntu
, then Docker
will default to using the ubuntu:latest
image.
Tip: We recommend you always use a specific tagged image, for example
ubuntu:12.04
. That way you always know exactly what variant of an image is being used.
Getting a new image
So how do we get new images? Well Docker will automatically download any image
we use that isn't already present on the Docker host. But this can potentially
add some time to the launch of a container. If we want to pre-load an image we
can download it using the docker pull
command. Let's say we'd like to
download the centos
image.
$ docker pull centos
Pulling repository centos
b7de3133ff98: Pulling dependent layers
5cc9e91966f7: Pulling fs layer
511136ea3c5a: Download complete
ef52fb1fe610: Download complete
. . .
Status: Downloaded newer image for centos
We can see that each layer of the image has been pulled down and now we can run a container from this image and we won't have to wait to download the image.
$ docker run -t -i centos /bin/bash
bash-4.1#
Finding images
One of the features of Docker is that a lot of people have created Docker images for a variety of purposes. Many of these have been uploaded to Docker Hub. We can search these images on the Docker Hub website.
We can also search for images on the command line using the docker search
command. Let's say our team wants an image with Ruby and Sinatra installed on
which to do our web application development. We can search for a suitable image
by using the docker search
command to find all the images that contain the
term sinatra
.
$ docker search sinatra
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
training/sinatra Sinatra training image 0 [OK]
marceldegraaf/sinatra Sinatra test app 0
mattwarren/docker-sinatra-demo 0 [OK]
luisbebop/docker-sinatra-hello-world 0 [OK]
bmorearty/handson-sinatra handson-ruby + Sinatra for Hands on with D... 0
subwiz/sinatra 0
bmorearty/sinatra 0
. . .
We can see we've returned a lot of images that use the term sinatra
. We've
returned a list of image names, descriptions, Stars (which measure the social
popularity of images - if a user likes an image then they can "star" it), and
the Official and Automated build statuses.
Official Repositories are a carefully curated set
of Docker repositories supported by Docker, Inc. Automated repositories are
Automated Builds that allow you to
validate the source and content of an image.
We've reviewed the images available to use and we decided to use the
training/sinatra
image. So far we've seen two types of images repositories,
images like ubuntu
, which are called base or root images. These base images
are provided by Docker Inc and are built, validated and supported. These can be
identified by their single word names.
We've also seen user images, for example the training/sinatra
image we've
chosen. A user image belongs to a member of the Docker community and is built
and maintained by them. You can identify user images as they are always
prefixed with the user name, here training
, of the user that created them.
Pulling our image
We've identified a suitable image, training/sinatra
, and now we can download it using the docker pull
command.
$ docker pull training/sinatra
The team can now use this image by running their own containers.
$ docker run -t -i training/sinatra /bin/bash
root@a8cb6ce02d85:/#
Creating our own images
The team has found the training/sinatra
image pretty useful but it's not quite what
they need and we need to make some changes to it. There are two ways we can
update and create images.
- We can update a container created from an image and commit the results to an image.
- We can use a
Dockerfile
to specify instructions to create an image.
Updating and committing an image
To update an image we first need to create a container from the image we'd like to update.
$ docker run -t -i training/sinatra /bin/bash
root@0b2616b0e5a8:/#
Note: Take note of the container ID that has been created,
0b2616b0e5a8
, as we'll need it in a moment.
Inside our running container let's add the json
gem.
root@0b2616b0e5a8:/# gem install json
Once this has completed let's exit our container using the exit
command.
Now we have a container with the change we want to make. We can then
commit a copy of this container to an image using the docker commit
command.
$ docker commit -m "Added json gem" -a "Kate Smith" \
0b2616b0e5a8 ouruser/sinatra:v2
4f177bd27a9ff0f6dc2a830403925b5360bfe0b93d476f7fc3231110e7f71b1c
Here we've used the docker commit
command. We've specified two flags: -m
and -a
. The -m
flag allows us to specify a commit message, much like you
would with a commit on a version control system. The -a
flag allows us to
specify an author for our update.
We've also specified the container we want to create this new image from,
0b2616b0e5a8
(the ID we recorded earlier) and we've specified a target for
the image:
ouruser/sinatra:v2
Let's break this target down. It consists of a new user, ouruser
, that we're
writing this image to. We've also specified the name of the image, here we're
keeping the original image name sinatra
. Finally we're specifying a tag for
the image: v2
.
We can then look at our new ouruser/sinatra
image using the docker images
command.
$ docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
training/sinatra latest 5bc342fa0b91 10 hours ago 446.7 MB
ouruser/sinatra v2 3c59e02ddd1a 10 hours ago 446.7 MB
ouruser/sinatra latest 5db5f8471261 10 hours ago 446.7 MB
To use our new image to create a container we can then:
$ docker run -t -i ouruser/sinatra:v2 /bin/bash
root@78e82f680994:/#
Building an image from a Dockerfile
Using the docker commit
command is a pretty simple way of extending an image
but it's a bit cumbersome and it's not easy to share a development process for
images amongst a team. Instead we can use a new command, docker build
, to
build new images from scratch.
To do this we create a Dockerfile
that contains a set of instructions that
tell Docker how to build our image.
Let's create a directory and a Dockerfile
first.
$ mkdir sinatra
$ cd sinatra
$ touch Dockerfile
If you are using Docker Machine on Windows, you may access your host
directory by cd
to /c/Users/your_user_name
.
Each instruction creates a new layer of the image. Let's look at a simple example now for building our own Sinatra image for our development team.
# This is a comment
FROM ubuntu:14.04
MAINTAINER Kate Smith <ksmith@example.com>
RUN apt-get update && apt-get install -y ruby ruby-dev
RUN gem install sinatra
Let's look at what our Dockerfile
does. Each instruction prefixes a statement and is capitalized.
INSTRUCTION statement
Note: We use
#
to indicate a comment
The first instruction FROM
tells Docker what the source of our image is, in
this case we're basing our new image on an Ubuntu 14.04 image.
Next we use the MAINTAINER
instruction to specify who maintains our new image.
Lastly, we've specified two RUN
instructions. A RUN
instruction executes
a command inside the image, for example installing a package. Here we're
updating our APT cache, installing Ruby and RubyGems and then installing the
Sinatra gem.
Note: There are a lot more instructions available to us in a Dockerfile.
Now let's take our Dockerfile
and use the docker build
command to build an image.
$ docker build -t ouruser/sinatra:v2 .
Sending build context to Docker daemon 2.048 kB
Sending build context to Docker daemon
Step 0 : FROM ubuntu:14.04
---> e54ca5efa2e9
Step 1 : MAINTAINER Kate Smith <ksmith@example.com>
---> Using cache
---> 851baf55332b
Step 2 : RUN apt-get update && apt-get install -y ruby ruby-dev
---> Running in 3a2558904e9b
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---> c55c31703134
Removing intermediate container 3a2558904e9b
Step 3 : RUN gem install sinatra
---> Running in 6b81cb6313e5
unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping
unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping
Successfully installed rack-1.5.2
Successfully installed tilt-1.4.1
Successfully installed rack-protection-1.5.3
Successfully installed sinatra-1.4.5
4 gems installed
Installing ri documentation for rack-1.5.2...
Installing ri documentation for tilt-1.4.1...
Installing ri documentation for rack-protection-1.5.3...
Installing ri documentation for sinatra-1.4.5...
Installing RDoc documentation for rack-1.5.2...
Installing RDoc documentation for tilt-1.4.1...
Installing RDoc documentation for rack-protection-1.5.3...
Installing RDoc documentation for sinatra-1.4.5...
---> 97feabe5d2ed
Removing intermediate container 6b81cb6313e5
Successfully built 97feabe5d2ed
We've specified our docker build
command and used the -t
flag to identify
our new image as belonging to the user ouruser
, the repository name sinatra
and given it the tag v2
.
We've also specified the location of our Dockerfile
using the .
to
indicate a Dockerfile
in the current directory.
Note: You can also specify a path to a
Dockerfile
.
Now we can see the build process at work. The first thing Docker does is upload the build context: basically the contents of the directory you're building in. This is done because the Docker daemon does the actual build of the image and it needs the local context to do it.
Next we can see each instruction in the Dockerfile
being executed
step-by-step. We can see that each step creates a new container, runs
the instruction inside that container and then commits that change -
just like the docker commit
work flow we saw earlier. When all the
instructions have executed we're left with the 97feabe5d2ed
image
(also helpfully tagged as ouruser/sinatra:v2
) and all intermediate
containers will get removed to clean things up.
Note: An image can't have more than 127 layers regardless of the storage driver. This limitation is set globally to encourage optimization of the overall size of images.
We can then create a container from our new image.
$ docker run -t -i ouruser/sinatra:v2 /bin/bash
root@8196968dac35:/#
Note: This is just a brief introduction to creating images. We've skipped a whole bunch of other instructions that you can use. We'll see more of those instructions in later sections of the Guide or you can refer to the
Dockerfile
reference for a detailed description and examples of every instruction. To help you write a clear, readable, maintainableDockerfile
, we've also written aDockerfile
Best Practices guide.
More
To learn more, check out the Dockerfile tutorial.
Setting tags on an image
You can also add a tag to an existing image after you commit or build it. We
can do this using the docker tag
command. Let's add a new tag to our
ouruser/sinatra
image.
$ docker tag 5db5f8471261 ouruser/sinatra:devel
The docker tag
command takes the ID of the image, here 5db5f8471261
, and our
user name, the repository name and the new tag.
Let's see our new tag using the docker images
command.
$ docker images ouruser/sinatra
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
ouruser/sinatra latest 5db5f8471261 11 hours ago 446.7 MB
ouruser/sinatra devel 5db5f8471261 11 hours ago 446.7 MB
ouruser/sinatra v2 5db5f8471261 11 hours ago 446.7 MB
Image Digests
Images that use the v2 or later format have a content-addressable identifier
called a digest
. As long as the input used to generate the image is
unchanged, the digest value is predictable. To list image digest values, use
the --digests
flag:
$ docker images --digests | head
REPOSITORY TAG DIGEST IMAGE ID CREATED VIRTUAL SIZE
ouruser/sinatra latest sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf 5db5f8471261 11 hours ago 446.7 MB
When pushing or pulling to a 2.0 registry, the push
or pull
command
output includes the image digest. You can pull
using a digest value.
$ docker pull ouruser/sinatra@cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf
You can also reference by digest in create
, run
, and rmi
commands, as well as the
FROM
image reference in a Dockerfile.
Push an image to Docker Hub
Once you've built or created a new image you can push it to Docker
Hub using the docker push
command. This
allows you to share it with others, either publicly, or push it into a
private repository.
$ docker push ouruser/sinatra
The push refers to a repository [ouruser/sinatra] (len: 1)
Sending image list
Pushing repository ouruser/sinatra (3 tags)
. . .
Remove an image from the host
You can also remove images on your Docker host in a way similar to
containers using the docker rmi
command.
Let's delete the training/sinatra
image as we don't need it anymore.
$ docker rmi training/sinatra
Untagged: training/sinatra:latest
Deleted: 5bc342fa0b91cabf65246837015197eecfa24b2213ed6a51a8974ae250fedd8d
Deleted: ed0fffdcdae5eb2c3a55549857a8be7fc8bc4241fb19ad714364cbfd7a56b22f
Deleted: 5c58979d73ae448df5af1d8142436d81116187a7633082650549c52c3a2418f0
Note: In order to remove an image from the host, please make sure that there are no containers actively based on it.
Next steps
Until now we've seen how to build individual applications inside Docker containers. Now learn how to build whole application stacks with Docker by linking together multiple Docker containers.
Test your Dockerfile knowledge with the Dockerfile tutorial.
Go to Linking Containers Together.