feat(docker): multi-arch Dockerfile (#668)
feat(docker): multi-arch Dockerfile (#668)

Adds an entirely new Dockerfile to the repository root which is capable of:

- building promtail and loki from the same image
- based on `scratch`, so that the final image can be executed on every supported
  GOARCH

Please note that this Dockerfile should always be built using
BuildKit (`buildctl` or `DOCKER_BUILDKIT=1` in recent versions) for maximum
performance, as BuildKit's DAG allows for smart skipping of uneeded stages.

These changes were proposed in #659
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki