Container security hardening (phase 4) (#24140)

* Support read-only rootfs with self-signed certs in /config/tls

* Support read-only rootfs in s6 and pre-compile bytecode

* Assert read-only rootfs support in CI

* Document hardened read-only deployment

* keep certsync's cert selection identical to nginx's

* note the uid trade-off in user: mode

* fail fast when EXTRA_GROUPS or a missing media volume meets read_only

* keep nosuid and nodev on the /run tmpfs

* support read_only in the default mode

* don't take go2rtc down when the homekit file isn't writable

* lead with the hardware consequence of switching to user:

* refuse to write TLS material through a symlink as root

* note that memryx writes models to the root filesystem

* certsync watches whichever cert path nginx loaded

docker/main/rootfs/etc/s6-overlay/s6-rc.d/certsync/run

@@ -26,7 +26,15 @@ function reload_nginx() {
 
 echo "[INFO] Starting certsync..."
 
-lefile="/etc/letsencrypt/live/frigate/fullchain.pem"
+# Resolved once, and the condition must stay identical to the nginx run
+# script's. Testing only fullchain.pem here would pick the mounted cert on a
+# half-populated mount that nginx rejected, and the two fingerprints would then
+# never agree, reloading nginx every cycle forever.
+if [ -f /etc/letsencrypt/live/frigate/privkey.pem ] && [ -f /etc/letsencrypt/live/frigate/fullchain.pem ]; then
+    lefile="/etc/letsencrypt/live/frigate/fullchain.pem"
+else
+    lefile="/config/tls/fullchain.pem"
+fi
 
 tls_enabled=`python3 /usr/local/nginx/get_nginx_settings.py | jq -r .tls.enabled`
 listen_external_port=`python3 /usr/local/nginx/get_nginx_settings.py | jq -r .listen.external_port`