[8.x] Bug fixes and minor improvements.

Added

• ❗❗❗ Added Doctor doing pre-flight checks before, during and after build. This makes working with the stack and resolving issues easier.
• ❗❗❗ Added support for PHPUnit tests + updated to PHPUnit 6.5 (used by Drupal core).
• Added LAGOON_PROJECT variable to handle cases when project in Lagoon is named differently then git repo.
• Added default user under which the containers should run. Allows to better run in Linux.

Fixed

• Fixed default Lagoon drush stub URL.
• Fixed handling of host volumes in tests.

Updated

• Updated dependencies to the latest versions.