Multi-platform build (linux/amd64 and linux/arm64)

[kokes]

Noticed that the official image is only amd64, so I added arm64 support (for AWS Graviton, M-series Macs etc.).

Tested:

• docker build -t foo . and docker build --platform linux/amd64 -t foo2 . and then docker inspect {foo,foo2} | grep -i arch
• disabled some GHCR bits from the action and tested the Action on my fork - it compiled - haven't checked it can get pushed to ghcr, but wouldn't expect it to fail - https://github.com/kokes/proxy/actions/runs/31098421797/job/92605876313 - it's not as slow as I'd think it'd be
• saw that there's Sign published container images #227, which might conflict with this a tiny bit (buildx), happy to rebase if you want to merge that one first