Allow overriding tags

[Firehed]

If running this action outside of the context of push or pull_request (generally, any situation where you may have run actions/checkout with a ref override), the image tags may have an undesirable value - I've observed this with a workflow_dispatch trigger where the ref is an input parameter.

I'm not entirely sure that providing the value is the correct behavior, beyond possibly trying to improve the commit inference in the action (e.g. run actual git commands). However this is still useful if the desired tag format doesn't match an an existing build convention, etc (e.g. using short commits)