Start Supervisor container with its cidfile mounted to /run/cid (#4276)

Use the --cidfile Docker CLI argument when starting the container and
bind-mount the generated file containing full ID of the container to the
container itself.

Using --mount instead of --volume is needed, as --volume is racy and creates
empty directory volume at the destination path instead.

This is prerequisite for home-assistant/supervisor#6006 but can come handy for
other cases too.

buildroot-external/rootfs-overlay/usr/sbin/hassos-supervisor

@@ -11,6 +11,7 @@ set -e
 # Init supervisor
 SUPERVISOR_DATA=/mnt/data/supervisor
 SUPERVISOR_STARTUP_MARKER="/run/supervisor/startup-marker"
+SUPERVISOR_CIDFILE="${SUPERVISOR_DATA}/hassio_supervisor.cid"
 SUPERVISOR_STARTSCRIPT_VERSION="/mnt/data/.hassos-supervisor-version"
 SUPERVISOR_IMAGE="ghcr.io/home-assistant/${SUPERVISOR_ARCH}-hassio-supervisor"
 
@@ -87,12 +88,18 @@ fi
 
 # If Supervisor container is missing, create it
 if [ -z "${SUPERVISOR_CONTAINER_ID}" ]; then
+    # We need to remove the CID file here, Docker will refuse to start if the
+    # file is present. Single instance is ensured by other code paths.
+    [ -f "${SUPERVISOR_CIDFILE}" ] && rm -f "${SUPERVISOR_CIDFILE}"
+
     echo "[INFO] Creating a new Supervisor container..."
     # shellcheck disable=SC2086
     docker container create \
         --name hassio_supervisor \
         --privileged --security-opt apparmor="hassio-supervisor" \
         --oom-score-adj=-300 \
+        --cidfile "${SUPERVISOR_CIDFILE}" \
+        --mount type=bind,src="${SUPERVISOR_CIDFILE}",dst=/run/cid,readonly \
         -v /run/docker.sock:/run/docker.sock:rw \
         -v /run/containerd/containerd.sock:/run/containerd/containerd.sock:rw \
         -v /run/systemd/journal/socket:/run/systemd/journal/socket:rw \