Follow up installer fixes (#621)

* enable installer on docker first run

* add SESSION_COOKIE to compose file

* `APP_ENVIRONMENT_ONLY` is long gone

* session env vars no longer needed after #624

* set defaults to null if sqlite is selected

.github/docker/entrypoint.sh

@@ -21,6 +21,9 @@ else
     echo -e "APP_KEY exists in environment, using that."
     echo -e "APP_KEY=$APP_KEY" > /pelican-data/.env
   fi
+
+  ## enable installer
+  echo -e "APP_INSTALLED=false" >> /pelican-data/.env
 fi
 
 mkdir /pelican-data/database