Publish images on GHCR. Closes #10

[regulartim]

Description

This adds a workflow for publishing container images on GHCR. Basically an adapted copy of the workflow we use in GreedyBear.

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[regulartim]

I can only really test if this works when it is merged into develop. :/

[opbot-xd]

Hi Tim! Thanks for the help here, was not very sure with docker publishing. The PR looks good and matches the faithful patterns of greedybear. :D