Unify Dockerfiles with Prom fork pattern (vendor + built split)

[bwplotka]

To ensure we test docker building step as close possible to internal CI, let's port here (and AM) what we did in Prometheus fork so:

• Dockerfile split into separate vendor and build steps https://github.com/GoogleCloudPlatform/prometheus/blob/10ff5b96bd822da7c1477fc796f4b4b19f88b8cc/Dockerfile.google#L36
• We use exactly same steps to build image as internally using buildx https://github.com/GoogleCloudPlatform/prometheus/blob/10ff5b96bd822da7c1477fc796f4b4b19f88b8cc/.github/workflows/presubmit.yml#L55