[codex] Add Docker testbed procedure

docs/README.md

@@ -90,6 +90,9 @@ daemon, provider, auth, deployment, or write-mode behavior.
   tests, browser smoke tests, live-Docker tests, and manual release smoke.
 - [testing/SANDBOX_FIXTURE.md](testing/SANDBOX_FIXTURE.md): isolated labeled
   Docker and provider-stub topology for manual DockerMap testing.
+- [testing/TESTBED_DOCKER_PROCEDURE.md](testing/TESTBED_DOCKER_PROCEDURE.md):
+  exact testbed startup, API/UI checks, automated commands, failure capture, and
+  teardown procedure.
 - [release/RELEASE_CHECKLIST.md](release/RELEASE_CHECKLIST.md): private alpha
   release gate and evidence to capture.
 - [design/DESIGN.md](design/DESIGN.md): UI direction.

docs/testing/SANDBOX_FIXTURE.md

@@ -4,6 +4,9 @@ Use the sandbox fixture when you want DockerMap to inspect a realistic but isola
 topology. It creates a labeled Docker Compose project, temporary provider command stubs,
 and a bounded project root under `/tmp`.
 
+For the full operator runbook, use
+[TESTBED_DOCKER_PROCEDURE.md](TESTBED_DOCKER_PROCEDURE.md).
+
 ## Start
 
 ```bash