Docker Images
We’ve looked at images before, but in this section we’ll dive deeper into what Docker images are and build our own image! Lastly, we’ll also use that image to run our application locally and finally deploy on AWS to share it with our friends! Excited? Great! Let’s get started.
Docker images are the basis of containers. In the previous example, we pulled the Busybox image from the registry and asked the Docker client to run a container based on that image. To see the list of images that are available locally, use the docker images
command.
$ docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
prakhar1989/catnip latest c7ffb5626a50 2 hours ago 697.9 MB
prakhar1989/static-site latest b270625a1631 21 hours ago 133.9 MB
python 3-onbuild cf4002b2c383 5 days ago 688.8 MB
martin/docker-cleanup-volumes latest b42990daaca2 7 weeks ago 22.14 MB
ubuntu latest e9ae3c220b23 7 weeks ago 187.9 MB
busybox latest c51f86c28340 9 weeks ago 1.109 MB
hello-world latest 0a6ba66e537a 11 weeks ago 960 B
The above gives a list of images that I’ve pulled from the registry, along with ones that I’ve created myself (we’ll shortly see how). The TAG
refers to a particular snapshot of the image and the IMAGE ID
is the corresponding unique identifier for that image.
For simplicity, you can think of an image akin to a git repository - images can be committed with changes and have multiple versions. If you don’t provide a specific version number, the client defaults to latest
. For example, you can pull a specific version of ubuntu
image
$ docker pull ubuntu:18.04
To get a new Docker image you can either get it from a registry (such as the Docker Hub) or create your own. There are tens of thousands of images available on Docker Hub. You can also search for images directly from the command line using docker search
.
An important distinction to be aware of when it comes to images is the difference between base and child images.
-
Base images are images that have no parent image, usually images with an OS like ubuntu, busybox or debian.
-
Child images are images that build on base images and add additional functionality.
Then there are official and user images, which can be both base and child images.
-
Official images are images that are officially maintained and supported by the folks at Docker. These are typically one word long. In the list of images above, the
python
,ubuntu
,busybox
andhello-world
images are official images. -
User images are images created and shared by users like you and me. They build on base images and add additional functionality. Typically, these are formatted as
user/image-name
.