Make Docker (and other?) actions fork-safe

[jhkennedy]

The docker build action always tries to push to GHCR, and thus will fail for forks as they don't have permission to push to GHCR. We should either figure out how to allow pushes from forks (maybe not the best idea from a sec perspective!) or skip the push action for forks.

[jhkennedy]

Fixed in #57

[jtherrmann]

Example of pushing and tagging successfully (on a non-fork repo): https://github.com/ASFHyP3/hyp3-ci/actions/runs/4388321365/jobs/7684692901

[jtherrmann]

I merged a PR from my fork of hyp3-ci but it looks like there are still failing actions: https://github.com/ASFHyP3/hyp3-ci/actions

I'm not sure if I'm taking the correct steps to reproduce the original bug. I'm reopening this issue until we've confirmed that the fix works.

[jhkennedy]

Confirm it does work as expected here: https://github.com/ASFHyP3/hyp3-ci/pull/7

If you use the GITHUB_TOKEN; will update the README