Move to the python:alpine docker image

- Remove `function` keyword, unsupported in shell

{{cookiecutter.project_slug}}/compose/production/django/entrypoint.sh

@@ -25,7 +25,7 @@ export DATABASE_URL=postgres://$POSTGRES_USER:$POSTGRES_PASSWORD@postgres:5432/$
 export CELERY_BROKER_URL=$REDIS_URL/0
 {% endif %}
 
-function postgres_ready(){
+postgres_ready() {
 python << END
 import sys
 import psycopg2