Create latest tag for commits on a specific branch

[pganster]

As far as I can see there are two cases in which the latest tag is applied:

1. latest=auto, for commits that have a git tag
2. latest=true, for every single commit on any branch

Another option, that would be useful, is where the latest tag is pushed on every single commit on a selected branch. E.g. latest=branch,main, like it's done with type=edge,branch=main.

[crazy-max]

@pganster #112 (comment) should do it.

[pganster]

Works like a charm, thanks a lot.

A possibility would be to add that example to the README file for future references so we know the flexibility of the flavor option.