doc: docker minimal setup

[Tubelight30]

I added a minimal PostgreSQL-in-Docker setup to CONTRIBUTING.md so someone can run go test ./... without installing PostgreSQL locally or using the devcontainer.
When I tried to set up the repo with a simple Docker Postgres container, I ran into two things that can be confusing for first-time contributors:

• go test can show (cached) even if the external DB environment changed, so I documented using go test -count=1 ./... when validating DB-related tests.
• Docker Postgres images often default to TimeZone=UTC while the Go process uses the host timezone (time.Local), which can cause timestamptz round-trip comparisons to fail. I documented running tests in UTC with TZ=UTC as a simple, deterministic workaround.

[jackc]

Docker Postgres images often default to TimeZone=UTC while the Go process uses the host timezone (time.Local), which can cause timestamptz round-trip comparisons to fail. I documented running tests in UTC with TZ=UTC as a simple, deterministic workaround.

That sounds like a test bug that should be fixed rather than documenting a workaround.