Add pin command

[MeneDev]

The pin command replaces image references with their equivalent, but more reproducible counterpart.
Ways to archive that:

• Add digest to image reference
• Add (more specific) tag.
  • none -> latest
  • none -> 1.2.1
  • latest -> 1.2.1
  • 1.2 -> 1.2.1

Note that the substitution should only be performed if the new reference is equivalent, i.e. referring to the same image.
Depending on the scope this can mean one of two things:

• Docker Daemon (blocked by Communicate with Docker Daemon to find current Digest of an image #1 )
• Docker Registry (blocked Communicate with Docker Registry to find current Digest of an image #2)

There should be an option to toggle between both scopes. Blocked by #8