Parallel multi-arch builds

In order to optimize the CI pipeline duration, we should consider running arch-specific builds in parallel – currently, those are built synchronously via the `docker-image/pyroscope/` Makefile build targets.

This would complicate the CI workflow ([example](https://docs.docker.com/build/ci/github-actions/multi-platform/#distribute-build-across-multiple-runners)) and is unlikely to speed up the builds, as the duration is primarily determined by the tests. Moreover, for releases, we use goreleaser, which cannot run in parallel in GitHub Actions.

That said, I am creating this issue for posterity.