Summary:
- Updates all Python 3.6, 3.7, 3.8 to the latest patch versions
- Move all images to AWS ECR region `us-west-2` (although note that ECR does have cross-region replication) - this is where the new Buildkite queues live. Moving because `us-west-1` does not support EKS / Kubernetes.
- Remove separate snapshot building and use multi stage builds for snapshots. This will make it straightforward to bring back the integration image building Buildkite pipeline rather than having to do it locally.
- Remove the Debian/Java nonsense we had to use when we were on both stretch/buster—since dropping 2.7 and 3.5 we're now 100% on buster, so the Java installation is much simpler.
- Renames for consistency and to differentiate Buildkite image repositories from others:
    - `test-image-builder` to `buildkite-test-image-builder`
    - `dagster-docker-buildkite` to `buildkite-test-image`
    - `dagster-core-docker-buildkite` to `buildkite-test-image-core`

Depends on D5776

Test Plan: integration

Reviewers: alangenfeld, dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D5771