Check and wait for a service in any linux distro

wait-for-a-service.sh

#!/bin/sh
# $: means that is an ENV variable

if [ "$DATABASE" = "postgres" ]
then
    echo "Waiting for postgres..."

    while ! nc -z $POSTGRESQL_ADDON_HOST $POSTGRESQL_ADDON_PORT; do
      sleep 0.1
    done

    echo "PostgreSQL started"
fi

python manage.py flush --no-input
python manage.py makemigrations
python manage.py migrate

exec "$@"

You need Netcat to run this successfully.