git tags do not currently work as image tags

[tazjin]

As pointed out by @eonpatapon in #86:

And I found in the manual for builtins.fetchGit: By default, the ref value is prefixed with refs/heads/. As of Nix 2.3.0 Nix will not prefix refs/heads/ if ref starts with refs/.
So that means that nixery would need to know if the requested docker tag is a git tag or git branch of the current repo before calling fetchGit. Which is kind of tricky since you can have a tag and a branch that have the same name.