Allow tags to be overwritten

[jpreese]

I currently use lstags for syncing. It supports all kinds of fancy regexes for defining versions (but of course they come with their own caveats)
Maybe supporting a list of tags for syncing images would come handy as well (do not know how to handle digest, though):

target:
  host: myhost.com
  repository: my/repo
images:
- repository: library/debian
  host: registry.hub.docker.com
  tag:
   - stretch
   - stretch-slim
   - buster
   - buster-slim
   - testing
   - sid

Originally posted by @mfriedenhagen in #8 (comment)

To add:

• Allow a single tag to be specified or an array of tags.
• Add a new attribute (overwrite:true ?) that will overwrite the tag at the target if the tag already exists.

[mfriedenhagen]

I would go for the following:

• Make tags a list of a map from name and digest.
• if digest is not given, assume this is a moving tag
• this ensures that no one is able to fiddle behind your back with the image unexpectedly