Draft: Optional MariaDB deployment instead of PostgreSQL deployment

I need a MariaDB deployment for an app instead of a PostgreSQL deployment. This MR attempts to adapt the auto deploy image / chart accordingly.

In .gitlab-ci.yml:

variables:
  POSTGRES_USER: user
  POSTGRES_PASSWORD: testing-password
  POSTGRES_ENABLED: "false"
  POSTGRES_DB: $CI_ENVIRONMENT_SLUG

  MARIADB_USER: user
  MARIADB_PASSWORD: testing-password
  MARIADB_ROOT_PASSWORD: testing-password
  MARIADB_ENABLED: "true"
  MARIADB_DB: $CI_ENVIRONMENT_SLUG

Deploy will fail if both are set to true. Pick one and stick with it.

• Update docs