Enable use of non-master branch as "latest"

[hugorodgerbrown]

From the tag_with_ref docs:

If {branch-name} is master then the tag will be latest.

We use a workflow that has separate master and dev branches, with strict rules around the interaction between them. Anything that requires a rebuild of the image (i.e. updating dependencies, rather than source files) must go through dev first. We have set dev as our main branch in Github, and it's locked, with controls around how devs can push to it.

We would like to have the latest tag always applied to the dev branch.

Appreciate we may be outliers here, but if it was possible to add a setting, it would really make a difference for us. I'll dig around myself and if I can see how to do it, will submit a PR.

[JakeWharton]

With the recent effort to migrate away from master in both git and GitHub this seems more important than ever. I'm currently using trunk for my primary integration branch and struggling with this.