Doesn't always use previous build cache

[corradio]

Steps to reproduce:

• brick build
• do a new commit
• brick build
• git revert that commit
• brick build
  Expected:
  The first brick build should have produced a cache entry, and thus the last brick build (after reverting) should re-use that cache and cause a no-op

[skovhus]

What happens here is that the second brick build overrides the tag. Docker layers are still cached, so it is rather fast right now.

The only way I see this could be improved is by tagging with branch+commit, which will bloat the docker cache.

If we solve output collection and image caching on branches (#57), then I believe Brick should be fast enough.

I'm closing this for now, please reopen if you disagree.