feat: make registry_token variable optional

[Kerwood]

The registry_token variable (action.yml#L20) is required for this action to run.

I am unsure as to where the environment variable GH_TOKEN is actually used and how it is authenticating. But its an issue if you want to use other image registries than ghcr.oi and need to use docker-credential-helpers, eg. for authenticating to Google Artifact Registry.

My proposal is to make registry_token optional and still make the bluebuild build command push the image but not authenticate if the registry_token is not set.

That way you could add a docker-credential-helper and docker login step before the blue-build action which would enable the use of other registries.