docs: add one-shot docker compose yaml to how-to

[chenba]

Add a one-shot docker file to start the db with docker compose also. Follow-up to STOR-118 / #1428 / PR #2019.

[taddes]

👍 Just one suggestion

[taddes]

I'll leave it up to you, but perhaps just a note to clarify that the hash value after syncstorage-rs-postgres: corresponds to a particular build in the aforementioned link (https://github.com/mozilla-services/syncstorage-rs/pkgs/container/syncstorage-rs%2Fsyncstorage-rs-postgres) and for someone to select that accordingly.

[chenba]

I was relying on the note at https://github.com/mozilla-services/syncstorage-rs/blob/master/docs/src/how-to/how-to-run-with-docker.md?plain=1#L15-L17 but yeah that is not exactly explicit. I can definitely mention the tag hash but there's no good, succinct advice on how to pick a tag; they are just merges on the main branch after all.

I was planning on updating the tags in the doc once we tagged the next release.

[pjenvey]

I think we should probably consider a separate issue to add additional docker tags to help with such setups, like a typical "latest", and "<major>", "<major>.<minor>" available on many of the popular dockers.

For now I'm fine with the hash, or how about syncstorage-rs-postgres:${SYNCSERVER_VERSION:-11659d98f9}