Add arm64 platform to container image build workflow

[spwoodcock]

Closes https://github.com/getodk/pyxform-http/issues/52

After merge, the images built via the Github workflow and published to ghcr.io should support AMD64 (default) and ARM64 also.

[lognaturel]

Sweet, thank you! 🚀

[spwoodcock]

Works nicely 😄