No `arm64` architecture image variant available

I'd like to run this on my RaspberryPi so I can `remote_write` Prometheus metrics to Azure Monitor. But I can't right now, because it appears an ARM64 image variant isn't currently being pushed.

It _should_ be as easy as adding `--platform linux/amd64,linux/arm64` to the `docker buildx build` command: https://github.com/Azure/aad-auth-proxy/blob/6c0f849997fa0d2ca3082b00710a2df130f31dc8/.pipelines/azure-pipeline-build.yml#L156

EDIT: Ah, I lied... Judging from your [`Dockerfile`](https://github.com/Azure/aad-auth-proxy/blob/main/src/Dockerfile#L17), it appears you're building the Golang binary externally and copying it into the final image. So I'm not exactly sure what all you'd need to do on your custom build system to enable this, but it sure would be appreciated! As of now, I'll have to fork this and build my own.