diff --git a/docker-compose.yml b/docker-compose.yml
index 8dd2018..8850fb1 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -132,6 +132,9 @@ services:
       - ./pack:/srv/pack:ro
       # Custom (locally-hosted) mod jars live OUTSIDE ./pack so packwiz refresh
       # doesn't index them as direct files. Served at pack.${BASE_DOMAIN}/custom/.
+      # Nested under the :ro /srv/pack mount — the mountpoint pack/custom/ must
+      # pre-exist on the host (tracked via pack/custom/.gitkeep) or Docker can't
+      # mkdir it in the read-only parent.
       - ./custom:/srv/pack/custom:ro
       # Static site from ANOTHER repo (absolute host path in .env).
       - ${DISTRIBUTION_WEB_ROOT:?DISTRIBUTION_WEB_ROOT unset in .env}:/srv/distribution:ro
diff --git a/pack/custom/.gitkeep b/pack/custom/.gitkeep
new file mode 100644
index 0000000..e69de29