feat: Basic implementation of docker_tag resource.

[Junkern]

This is introducing a new docker_tag resource which does the same thing as the docker tag command: It tags images.
The feature set is pretty basic:

• Have a source_image and tag it as target_image
• Run the tag command whenever any of those two values change
• We won't support deletion of any tagged images when the docker_tag resource is destroyed.

This will allow for many new workflows which were not really possible before (e.g. #137 )