Enable GitHub Actions for automated docker builds

[giffels]

Docker Hub's Autobuild service has been discontinued for free plans. This pull request enables automated docker builds using GitHub Actions as replacement and fixes #191.

• Docker builds of tardis tags/releases are tagged as latest and with the corresponding version (e.g. 0.7.0).
• Docker builds of the master branch are tagged as latest.
• On all other pull_request and push events images are build, but not pushed into the docker hub registry.

[codecov-commenter]

Codecov Report

Merging #203 (66940b5) into master (bd55dab) will not change coverage.
The diff coverage is n/a.

❗ Current head 66940b5 differs from pull request most recent head d49525f. Consider uploading reports for the commit d49525f to get more accurate results

@@           Coverage Diff           @@
##           master     #203   +/-   ##
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  Coverage   99.55%   99.55%           
=======================================
  Files          42       42           
  Lines        1795     1795           
=======================================
  Hits         1787     1787           
  Misses          8        8           

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[HerrHorizontal]

Looks fine to me.

[maxfischer2781]

Storing the script as a multi-line string seems more maintainable to me. I've added two possibilities; perhaps having an actual script inside the containers folder might be an idea as well?

[giffels]

Storing the script as a multi-line string seems more maintainable to me. I've added two possibilities; perhaps having an actual script inside the containers folder might be an idea as well?

Thanks for your suggestion. I took the pathlib variant.

[maxfischer2781]

● As I see it, yes. 🎱