docker-compose.yml now builds from source (dev-forward: run your local changes);
docker-compose.prod.yml overrides it to pull ghcr latest (build reset via !reset).
Adds make compose-up / compose-up-prod / compose-down. Env, ports, and extra_hosts
stay defined once in the base file; the prod override only swaps build -> image.
make smoke-docker builds the image, stands up a stub upstream + the container
on a shared network, and asserts forward+prefix-strip, upstream-header cleared,
421 on missing header, and 403 + reject sentinel on allowlist miss. Self-
contained (no tailnet), idempotent, local-only (Docker) like cargo-check.
