Add comment about Docker secrets to example.env (#2454)

Add a comment to indicate the support for Docker secrets added in https://github.com/immich-app/immich/pull/1254

docker/example.env

@@ -2,6 +2,8 @@
 # Database
 ###################################################################################
 
+# NOTE: The following four database variables support Docker secrets by adding a *_FILE suffix to the variable name
+# See the docker-compose documentation on secrets for additional details: https://docs.docker.com/compose/compose-file/compose-file-v3/#secrets
 DB_HOSTNAME=immich_postgres
 DB_USERNAME=postgres
 DB_PASSWORD=postgres