Use skopeo to tag images multiple times (!31)

Resolves #20 (closed)

Resolves #7 (closed)

c.f. https://github.com/containers/skopeo

The skopeo image used is minimal and built from the official golang Docker image.

Suggestion for this came from @lheinric: https://twitter.com/lukasheinrich_/status/1098314374207496194