feat: publish container images to GHCR

[rizlas]

Closes #106

Summary

Adds automated container image builds and publishing to GHCR (ghcr.io).

Notes

Docker Hub publishing is not included in this PR. Pushing to Docker Hub requires credentials that only maintainer has access to.

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[rizlas]

I've forced push to sign the previous commit and comply with DCO. Hope that his will make it to upstream @wrenix .

Thanks