don't follow a planted symlink when writing /dev/shm/go2rtc.yaml

After the first start the go2rtc user owned /dev/shm/go2rtc.yaml and could swap it for a dangling symlink. The run script's -f test was false for a dangling link, so the link survived, and root's create_config.py then created the link target and the chown handed it to go2rtc. The run script now removes any existing entry, create_config.py creates the file with O_EXCL and O_NOFOLLOW at mode 0640, and the chown uses -h.

docker/main/rootfs/usr/local/go2rtc/create_config.py

@@ -198,9 +198,13 @@ if config.get("birdseye", {}).get("restream", False):
     else:
         go2rtc_config["streams"] = {"birdseye": ffmpeg_cmd}
 
-# Write go2rtc_config to /dev/shm/go2rtc.yaml
-with open("/dev/shm/go2rtc.yaml", "w") as f:
+# The run script removes the old file first. Exclusive create never follows a
+# link the go2rtc user planted in sticky /dev/shm. 0640 because the config
+# contains camera credentials.
+fd = os.open(
+    "/dev/shm/go2rtc.yaml",
+    os.O_WRONLY | os.O_CREAT | os.O_EXCL | os.O_NOFOLLOW,
+    0o640,
+)
+with os.fdopen(fd, "w") as f:
     yaml.dump(go2rtc_config, f)
-
-# config contains camera credentials; do not leave it world-readable
-os.chmod("/dev/shm/go2rtc.yaml", 0o640)