Fix CI boot check take 3: \$\$ isn't unique across job containers

Every job container's entry shell gets the same low PID, so
backend-ci-\$\$ collided with an already-stuck leftover container from
a prior run (confirmed in the runner logs: "container name already in
use"). GITHUB_RUN_ID is genuinely unique per run -- use that for both
the container name and the per-run host port instead.

.forgejo/workflows/build.yml

@@ -35,14 +35,17 @@ jobs:
         # failing fast) -- confirmed by a stuck run. 127.0.0.1 DOES work
         # (proven by the monorepo's own build.yml, which has used a
         # published host port successfully all along), so publish one, but
-        # to a per-run PID-derived port rather than a fixed one -- this
-        # runner's snap-Docker install has a known AppArmor bug denying
-        # docker stop/kill, so a leftover container from any prior failed
-        # run permanently squats whatever host port it was given, and a
-        # fixed port would block every subsequent run too.
+        # to a per-run unique port rather than a fixed one -- this runner's
+        # snap-Docker install has a known AppArmor bug denying docker
+        # stop/kill, so a leftover container from any prior failed run
+        # permanently squats whatever host port it was given, and a fixed
+        # port would block every subsequent run too. GITHUB_RUN_ID (NOT
+        # $$ -- every job container's shell gets the same low PID, so that
+        # collided with an already-stuck leftover from a prior run) is
+        # genuinely unique per run.
         run: |
-          name="backend-ci-$$"
-          port=$((18000 + ($$ % 1000)))
+          name="backend-ci-${GITHUB_RUN_ID}"
+          port=$((18000 + (GITHUB_RUN_ID % 1000)))
           docker run -d --name "$name" \
             -e THERMOGRAPH_FRONTEND_BASE_INTERNAL=http://127.0.0.1:1 \
             -p "127.0.0.1:${port}:8137" thermograph-backend:ci