Interactive Image Creation

By the end of this exercise, you should be able to:

• Capture a container's filesystem state as a new docker image
• Read and understand the output of docker container diff

Modifying a Container

1. Start a bash terminal in a CentOS container:

   [centos@node-0 ~]$ docker container run -it centos:7 bash
   

2. Install a couple pieces of software in this container - there's nothing special about wget, any changes to the filesystem will do. Afterwards, exit the container:

   [root@dfe86ed42be9 /]# yum install -y which wget
   [root@dfe86ed42be9 /]# exit
   

3. Finally, try docker container diff to see what's changed about a container relative to its image; you'll need to get the container ID via docker container ls -a first:

   [centos@node-0 ~]$ docker container ls -a
   [centos@node-0 ~]$ docker container diff <container ID>
   
   C /root
   A /root/.bash_history
   C /usr
   C /usr/bin
   A /usr/bin/gsoelim
   ...
   

   Those Cs at the beginning of each line stand for files Changed, and A for Added; lines that start with D indicate Deletions.

Capturing Container State as an Image

1. Installing which and wget in the last step wrote information to the container's read/write layer; now let's save that read/write layer as a new read-only image layer in order to create a new image that reflects our additions, via the docker container commit:

   [centos@node-0 ~]$ docker container commit <container ID> myapp:1.0
   

2. Check that you can see your new image by listing all your images:

   [centos@node-0 ~]$ docker image ls
   
   REPOSITORY  TAG   IMAGE ID       CREATED         SIZE
   myapp       1.0   34f97e0b087b   8 seconds ago   300MB
   centos      7     5182e96772bf   44 hours ago    200MB
   

3. Create a container running bash using your new image, and check that vim and wget are installed:

   [centos@node-0 ~]$ docker container run -it myapp:1.0 bash
   [root@2ecb80c76853 /]# which wget
   

   The which commands should show the path to the specified executable, indicating they have been installed in the image.

Conclusion

In this exercise, you saw how to inspect the contents of a container's read / write layer with docker container diff, and commit those changes to a new image layer with docker container commit. Committing a container as an image in this fashion can be useful when developing an environment inside a container, when you want to capture that environment for reproduction elsewhere.