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Docker images

What is a docker image?

Images are multi-layered self-contained files that act as the template for creating containers. They are like a frozen, read-only copy of a container. Images can be exchanged through registries. Containers are just images in running state. When you obtain an image from the internet and run a container using that image, you essentially create another temporary writable layer on top of the previous read-only ones

Key image features

  • A docker image is a snapshot of the filesystem + some metadata
  • Identified by unique hex IDs
  • may be tagged(severally) with a human-friendly name

Managing Docker images

  1. Fetching an image from Dockerhub (docker pull)
  • Images are pulled from Docker hub
  • Docker initiates a pull of the latest image
  • You can pull a specific version using tags
  • You can also pull an image from a Registry

To pull a latest image

docker pull ubuntu

To pull a tagged image

docker pull ubuntu:14.04
docker pull registry.example.com/username/ubuntu:14.04

  1. Listing downloaded images

    docker images
  1. Building Docker images

You can create an image first by downloading a template or sample image from the docker repository.

For example

docker pull ubuntu

Secondly, Once you have a Dockerfile, you can build an image from it using docker build. The basic form of this command is:

docker build -t image-name /path
docker build -t image-name .  //building in the current directory
  1. Tagging Docker images

Tagging an image is useful for keeping track of different image versions:

  • Usage

    docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]

  • Example usage

    docker tag 6efc10a0510f:stablev1 nginx:stablev2
  1. Referencing Docker images
  • You can reference by image ID: for example 693bce725149
  • You can reference by Name: for example hello-world (defaults to :latest tag)
  • You can reference by Name + Tag: for example hello-world:latest
  1. Removing images The docker rmi command is used to remove images:
docker rmi  <image name>

It is also possible to remove images by their ID instead:

docker rmi 693bce725149

You can also remove the images using the first three characters of the image id

Remove All Images with No Started Containers

An image cannot be removed locally if it is being RUN by a container.

To remove all local images that have no started containers, you can provide a listing of the images as a parameter:

docker rmi $(docker images -qa)

If you want to remove images regardless of whether or not they have a started container use the force flag (-f):

docker rmi -f $(docker images -qa)
  1. Searching images

You can search Docker Hub for images by using the search command:

docker search <term>

For Example

docker search nginx
  1. Searching images

The output is in JSON format

docker inspect <image>
  1. Saving an image
docker save -o ubuntu.latest.tar ubuntu:latest

This command will save the ubuntu:latest image as a tarball archive in the current directory with the name ubuntu.latest.tar.

This tarball archive can then be moved to another host, for example using rsync, or archived in storage.

Once the tarball has been moved, the following command will create an image from the file

docker load -i /tmp/ubuntu.latest.tar
  1. Pushing and image to Dockerhub

Locally created images can be pushed to Docker Hub or any other docker repo host, known as a registry. Use docker login to sign in to an existing docker hub account.

docker login
  • Enter: Username
  • Enter: Password

You can then tag and push images to the registry that you are logged in to. Your repository must be specified as /username/reponame:tag.

For example

docker tag mynginx cjsimon/mynginx:latest