fix(tests): auto-cap docker-suite workers in the runner itself

Move the dockerd-bound worker cap from a workflow env var into
run_tests_parallel.py: when every file in the run is under
tests/docker/, cap -j at 4 (the suite shares one docker daemon; width
beyond that thrashes it — files stretch ~100s -> ~900s and teardown
docker-rm calls blow their 10s timeout). Explicit -j or
HERMES_TEST_WORKERS always wins; mixed file lists are unaffected.
Verified: docker-only list caps 32->4, -j 12 respected, mixed list
uncapped. Drops the HERMES_TEST_WORKERS=4 pin from docker.yml.

.github/workflows/docker.yml

@@ -119,15 +119,13 @@ jobs:
           command: uv sync --locked --python 3.11 --extra dev
 
       - name: Run docker integration tests
-        # HERMES_TEST_WORKERS=4: this suite shares ONE dockerd — parallel
-        # width is bounded by the daemon, not CPU. At cpu*2 workers (16 on
-        # the 8-CPU ARC pods) dockerd thrashes: every file takes ~900s and
-        # teardown `docker rm` blows its 10s timeout. arm64 (2 CPU → -j4)
-        # passes for exactly this reason.
+        # The test runner auto-caps workers for tests/docker (the suite is
+        # bounded by the single dind dockerd, not CPU — see
+        # run_tests_parallel.py's docker-suite auto-cap).
         uses: ./.github/actions/profile
         with:
           label: docker-tests
-          command: HERMES_TEST_WORKERS=4 scripts/run_tests.sh tests/docker/ --file-timeout 600
+          command: scripts/run_tests.sh tests/docker/ --file-timeout 600
 
   # ---------------------------------------------------------------------------
   # Rebuild and push each architecture only after the unprivileged build/test