Allows the CLI to work correctly when run inside a dev container with
the Docker socket bind-mounted. In that context 127.0.0.1 is the
container's own loopback, not the Docker host.

Set SUPABASE_HOSTNAME=host.docker.internal to reach sibling containers
via Docker Desktop's host gateway without proxying the Docker socket.

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AI assistance: Claude Sonnet 4.6 in Pi Agent
- Used to find root cause, propose fix options, draft code/test/docs.
- I ran supabase start inside the dev container, confirmed all services
  healthy, reviewed/edited all code changes/text.