Add GitHub Action for publishing pwnmake, pwncc, and base Docker images on push

[C0deH4cker]

As I'm almost done with support for easily building, tagging, and pushing Docker images for the pwnmake builder image and the base PwnableHarness image, it would be great to automatically push updated images to Docker Hub whenever a new commit is pushed. I'll need to be careful about when I update the version number.

Plan is to set it up as a GitHub Action using a self-hosted runner, and then only pushes to master (or merged PRs) will trigger the Actions runner.

[C0deH4cker]

All of the needed functionality for building, tagging, and pushing the various images (pwnmake, pwncc, and base) is implemented. This should be easy-ish now, just need to create the custom GitHub Action.