Feature: Add 'tag' input to allow creating tags based on upstream versions

I maintain several repositories where the docker repo is separate from the application repo. I wrote an action a while back HackerHappyHour/tagging-strategy that had a lot of the same intentions of this action, and I’d like to archive it in favor of this action.

The only feature my action has that isn’t yet implemented here is the ability to (optionally) supply a tag value, instead of getting the tag from the repository.

This allows upstream repo’s to dispatch on tagging/releases, and and for the docker repo to build/publish tags when that happens.

Example where we do this: github.com/OctoPrint/Octoprint triggers github.com/OctoPrint/octoprint-docker.

This goes especially well with #15, which was the other feature missing from this action, but seems like @hugopeixoto beat me to it!

I’d be willing to contribute this feature, just need approved suggestion on the input name. I’m thinking:

inputs:
  # other inputs
  input_tag:
    description: 'if provided will use the supplied value. Not compatible with all inputs (see README.md for compatibility)'
    required: false

Then for docs adding a 4th column in the inputs table in the README, specifying if compatible with input_tag, along with notes like

using input_tag is for scenarios when using external events to trigger builds is required such as repository_dispatch or workflow_dispatch

[crazy-max]

@LongLiveCHIEF

well yeah, that’s why i’m doing what I’m doing. 😃

Oh ok that’s a limitation of your action, got it!

[crazy-max]

@LongLiveCHIEF Sorry didn’t see the end of your post ahah