tagging imrovements

[errordeveloper]

Some feedback from @justincormack:

• multiple git tags should result in multiple images (perhaps this could be an opt-in or opt-out)
  • some option to ignore tags maybe needed
• -dirty is traditional suffix (currently imagine has -wip, for philosophical reasons, but perhaps that could be user-settable)
• use git tags for dir-scoped image also

[errordeveloper]

Non-semver tags should be also supported, but probably as an explicit mode.

[errordeveloper]

One additional point about multiple tags is that with repos that have multiple go.mod files, there will be per-module tags also.