How to create a user for Postgres from the command line for bash automation
This will create user admin with password test101 on localhost:
psql -c "CREATE USER admin WITH PASSWORD 'test101';"
To run it from any user just add the sudo -u postgres to it:
sudo -u postgres bash -c "psql -c \"CREATE USER vagrant WITH PASSWORD 'vagrant';\""
To run it on the original docker image for example you can use the su -c command with the postgres user in combination with psql:
su -c "psql -c \"CREATE ROLE my_user WITH LOGIN PASSWORD 'my_password' \"" postgres