Added 'Docker Login' Step back to support required cases

[pivotal-david-osullivan]

Summary

Removing the Docker Login step completely from 'Create Package Test' action caused failures for some composite buildpacks where it is required. This patch adds the step back for non Paketo buildpacks and also adds a check which will not run the Login step if the actor is Dependabot, since it will not have the required credentials to succeed(recommended by Github Support).

Use Cases

The Docker Login step will continue to fail for Dependabot PRs in some composite buildpacks, re-running the action should supply the credentials and the check should then pass.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[pivotal-david-osullivan]

@dmikusa-pivotal Could I get your eyes on this please before I cut a release, validate & roll out?
I had to change the conditional logic a bit to still support the old checks (e.g. no forks) and add the Dependabot actor check

[dmikusa]

The logic seems good to me. You have the same logic as before &&'d with is_not_dependabot. So every case that was previously true will only be true if the actor is not dependabot. Every case that was previously false will still be false regardless of the actor. Anything submitted by dependabot will always be false. I think that's what we want.