Add support for GitHub Actions build workflow on PRs

[victorpopkov]

Problem

Similar to dstmodders/docker-imagemagick#33:

This is similar to dstmodders/docker-imagemagick#23 with the solution provided in dstmodders/docker-imagemagick#26, and should pretty much match the on.push behaviour.

Of course, no pushes should be made into the public repository.

Proposed solution

See dstmodders/docker-imagemagick#26.

Progress

• Add permissions.
• Add concurrency.
• Add on.pull_request.branches.
• Add on.pull_request.paths.
• Change if statements, if necessary.
• Make sure that nothing gets pushed into the public repository on PRs.