wont let me build a branch name with "@" in it

[dalyons]

& I wanna.

[ejholmes]

Apparently this is a limitation of docker tags, which must match [a-z0-9]+(?:[._-][a-z0-9]+)*. We could disable the branch tag if it doesn't match this regex, but then we'll lose the caching advantage. Might be better to just error out with a better error message.

[ejholmes]

Another option would be to just use a hash of the branch name for the cache tag (but still push the branch for convenience, if it matches that regex).