ci: fix Docker container build

[fhunleth]

No description provided.

[jjcarstens]

Seems like this isn't quite fixed?

[fhunleth]

No, I pinged Digit for help.

[doawoo]

OK! @fhunleth @jjcarstens I think I have finally figured out the dark arts of creating a multi-arch docker image. Turns out you cannot just push one easily outside of using --push in the buildx sub-command. So I build them individually, push them as dev tags, then combine them into a single manifest which will act like a "unified" AMD64/ARM64 image when pulled.

NOTE: we cannot test the ARM image easily in CircleCI, they do not support Docker ARM execution very well, and, it would simply take forever...

[fhunleth]

@doawoo You're a hero! Thank you for figuring this out!!!!!