GHA docker

[mmacata]

This PR uses now GHA to build and push docker images to dockerhub.
It will also replace the separate branch for builds with grass79, so branch will be deleted in future: https://github.com/mundialis/actinia_core/compare/grass79
This way, no separate 79 release is necessary anymore - which is better, as the actinia code base was the same but needed to be released twice - for g78 and g79.

This workflow takes care of:

• build and push the following tags for changed in main branch:

  • "g78-latest-alpine"
  • "g78-latest-ubuntu"
  • "g79-latest-alpine"
  • "g79-latest-ubuntu"
  • "latest"
  • git ref tag, eg. "sha-ad132f5"

• build and push the following tags for releases:

  • update "latest" tag
  • release tag major.minor.path, e.g. "1.2.3" (alpine g78)
  • release tag major.minor, e.g. "1.2" (alpine g78)
  • release tag for g79, e.g. "1.2.3.79" (alpine g79)

[anikaweinmann]

As far as I understand the GHA it looks good for me

[mmacata]

All triggered builds were successful!
https://github.com/mundialis/actinia_core/runs/3313392581?check_suite_focus=true
all 6 tags were build and pushed within 12 minutes after merge 🚀