refactor: cleanup Dockerfile

1. Switches from standalone thanos asdf plugin to three-in-one, thanos, promtool and amtool plugin. Project details are here: https://gitlab.com/gitlab-com/gl-infra/asdf-promtool/.
2. Switches amtool (the alertmanager validation tool) from hardcoded version to .tool-versions/ASDF managed like everything else in this repository.
3. Pull binaries from other official images (where possible) and our own https://gitlab.com/gitlab-com/gl-infra/third-party-container-images/ project in some cases to DRY out building and installing binaries
4. Avoid downloading tarballs and switch to official images instead.
5. Allow renovate to automatically upgrade amtool versions, like we do for other tooling. A renovate version constrain has also been added, to ensure that we don't upgrade beyond the same minor version of alertmanager as is currently running in Production. See gitlab-com/gl-infra/common-ci-tasks!136 (merged) for details.
6. Switches from gcloud base image with ruby installed to a ruby base image with gcloud installed