PostgreSQL: Update README
PostgreSQL: Use `postgres` scheme

While both `postgresql` and `postgres` scheme are allowed,
`postgres` is used by Heroku.
Allow to run bootstrap.sh in non-interactive mode
Allow to suppress firewall warning with DOKKU_SUPPRESS_FIREWALL_WARNING=1 added to dokkurc
README: add section about dokkurc
Allow alternate postgresl image using dokkurc
Allow alternate redis image using dokkurc
Allow alternate mariadb image using dokkurc
Allow alternate mongodb image using dokkurc