fix(linux/build): find self via docker inspect, cgroupns hides cgroup path (M1.1 iter22)

iter21's /proc/self/cgroup approach hit:

    build.sh: cgroup contents:
    0::/

Empty path — act_runner runs job containers with cgroupns enabled, so
the in-container view of cgroup paths is rooted at the namespace, with
no trace of the host-side container ID. Same blocker as `hostname`.

The host docker daemon does know who we are, and we have its socket.
We're the only running container with /workspace/SilverLABS/SilverMetal
as a mount destination (concurrency: 1 in the workflow), so iterate
docker ps and match by mount destination. Found CID becomes the
--volumes-from argument; if no match, dump docker ps to the log and
fail loud.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

linux/build/scripts/build.sh

@@ -100,20 +100,33 @@ echo "build.sh: output -> ${BUILD_DIR}"
 # paths identical inside and outside, so the inner heredoc below is the
 # same in both environments.
 #
-# Discovering the job container's own ID: `hostname` is unreliable on
-# act_runner / catthehacker (returned the literal string "docker" once
-# the runner was running with config.yaml's `network: host` applied —
-# see run #4268). /proc/self/cgroup is the portable way:
-#   * cgroup v1: lines look like `12:devices:/docker/<64-hex>`
-#   * cgroup v2: `0::/system.slice/docker-<64-hex>.scope`
-# Either way the 64-char hex container ID is in the path. Extract the
-# first one.
+# Discovering the job container's own ID. Three "obvious" approaches
+# all fail for this runner setup:
+#   * `hostname` returns the literal string "docker" — catthehacker's
+#     /etc/hostname (run #4268).
+#   * /proc/self/cgroup returns just "0::/" because act_runner's job
+#     containers run with cgroupns enabled, hiding the host cgroup path
+#     (run #4269).
+#   * /proc/1/cpuset hits the same cgroupns wall.
+#
+# What we *do* have is the docker.sock pass-through. We're definitionally
+# the only running container with /workspace/SilverLABS/SilverMetal as
+# a mount destination (concurrency: 1 in build-iso-linux.yaml), so ask
+# the host daemon to find us by that.
 if [[ -n "${GITHUB_ACTIONS:-}" ]]; then
-    SELF_CID="$(awk 'match($0, /[a-f0-9]{64}/) { print substr($0, RSTART, RLENGTH); exit }' /proc/self/cgroup 2>/dev/null || true)"
+    SELF_CID=""
+    for cid in $(docker ps -q --no-trunc 2>/dev/null); do
+        if docker inspect "$cid" --format \
+            '{{range .Mounts}}{{if eq .Destination "/workspace/SilverLABS/SilverMetal"}}match{{end}}{{end}}' \
+            2>/dev/null | grep -q match; then
+            SELF_CID="$cid"
+            break
+        fi
+    done
     if [[ -z "${SELF_CID}" ]]; then
-        echo "build.sh: could not determine own container ID from /proc/self/cgroup" >&2
-        echo "build.sh: cgroup contents:" >&2
-        cat /proc/self/cgroup >&2 || true
+        echo "build.sh: could not find own container via docker inspect on /workspace mount" >&2
+        echo "build.sh: docker ps -q output:" >&2
+        docker ps --no-trunc >&2 || true
         exit 1
     fi
     echo "build.sh: --volumes-from ${SELF_CID:0:12}"