[docker] Fix docker-init missing FLASK_APP (apache#7709)

Author: dpgaspar

contrib/docker/docker-init.sh

@@ -18,6 +18,7 @@
 set -ex
 
 # Create an admin user (you will be prompted to set username, first and last name before setting a password)
+export FLASK_APP=superset:app
 flask fab create-admin
 
 # Initialize the database