Build multiarch protobuf Docker images

[yvrhdn]

This adapts protobuf/Dockerfile so it can be built for amd64 and arm64 targets by using TARGETARCH. I had to update PROTOC_GEN_LINT_VERSION as only v0.2.4 has downloads for all archs.

Next I updated the GitHub Actions workflow, I'm using docker/build-push-action to build multiarch images. Under the hood this uses docker buildx.

You can run buildx locally using:

docker buildx build --load --platform linux/arm64 -t otel/protobuf:latest protobuf

Fixes https://github.com/open-telemetry/build-tools/issues/42

[yvrhdn]

I see I used the wrong secret name, this should work 🙂

[bogdandrutu]

@kvrhdn please address all the concerns/comments.