Normalize compose host bind mount paths

.env.example

@@ -4,6 +4,9 @@
 # Development defaults (HTTP local stack)
 APP_ENV=development
 HOST_BIND_IP=127.0.0.1
+# Optional host directory for persistent bind mounts in docker-compose.yml.
+# Defaults to ./data when unset.
+# DCM_DATA_DIR=./data
 
 POSTGRES_USER=dcm
 POSTGRES_PASSWORD=ChangeMe-Postgres-Secret