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

Start a Powershell terminal in a Windows Server Core container:

PS: node-0 Administrator> docker container run `
    -it --name demo microsoft/windowsservercore powershell

Install a couple pieces of software in this container - First install a package manager; in this case Chocolatey:
```
PS C:\> iex (iwr https://chocolatey.org/install.ps1 -UseBasicParsing)
```
Then install some packages. There's nothing special about wget, any changes to the filesystem will do. Afterwards, exit the container:
```
PS C:\> choco install -y wget
PS C:\> exit
```
Finally, try docker container diff to see what's changed about a container relative to its image:
```
PS: node-0 Administrator> docker container diff demo

C Files
C Files/Documents and Settings
C Files/Program Files (x86)
...
```
Those Cs at the beginning of each line stand for files Changed; lines that start with D indicate Deletions.

Capturing Container State as an Image

Installing 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:
```
PS: node-0 Administrator> docker container commit demo myapp:1.0
```

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

PS: node-0 Administrator> docker image ls

REPOSITORY    TAG      IMAGE ID            CREATED             SIZE
myapp         1.0      9ce128f61c85        2 minutes ago       11GB
...

Create a container running Powershell using your new image, and check that wget is installed:

PS: node-0 Administrator> docker container run -it myapp:1.0 powershell
PS C:\> cd \ProgramData\chocolatey\lib
PS C:\> ls

    Directory: C:\ProgramData\chocolatey\lib

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----        8/29/2018  10:30 PM                chocolatey
d-----        8/29/2018  10:31 PM                Wget

The software you installed in your previous container is also available in this container, and all subsequent containers you start from the image you captured using docker container commit.

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.