Make the container run as root by default, if env variables are missing, to be backwards compatible to old setup

compose.yaml

@@ -5,7 +5,7 @@ services:
   shimatta-kenkyusho-web:
     <<: *restart_policy
     build: .
-    user: "${DJANGO_USER_ID}:${DJANGO_USER_GID}"
+    user: "${DJANGO_USER_ID:-0}:${DJANGO_USER_GID:-0}"
     volumes:
       - "${DJANGO_STATIC_VOL:-./run/static}:/var/static"
       - "${DJANGO_MEDIA_VOL:-./run/media}:/var/media"