Push image to registry only when building `master`

[dbirks]

I noticed the builds were failing recently. :x: :cry:
It looks like it's trying to always login to Docker Hub, even for PRs, which shouldn't need to push any image.

This PR adds a check to see if we're building off master, and if we're not, then it skips the docker login step and the push step.

Thanks for maintaining this!

[codecov[bot]]

Codecov Report

Merging #114 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #114   +/-   ##
=======================================
  Coverage   86.59%   86.59%           
=======================================
  Files           9        9           
  Lines         261      261           
=======================================
  Hits          226      226           
  Misses         26       26           
  Partials        9        9           

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[knelasevero]

Hey @dbirks, thanks a lot for the contribution! We actually want to keep pushing images internally, but stop doing that for forks. Do you think that you could change your check to actually test if it is a fork?

So, instead of checking if it is master, check if the repo full name is the one from the original org. Maybe something like if github.event.pull_request.head.repo.full_name == 'org/repo'.

[dbirks]

Ah, yes that sounds good. I was wondering how to prevent untrusted code from being built and pushed, but having the line drawn at forks would work very nicely, and allow you to have internal branches pushed. Making the change now...