optionally run git clone/fetch before starting the services

Description

Services in CI jobs may use the project code in $CI_PROJECT_DIR for bootstrapping (e.g., database migrations to be run in the database container). However git clone / fetch is executed only after the service containers have been started. As a result the services start with the obsolete state of $CI_PROJECT_DIR.

Proposal

Add an option to run git clone / fetch before the services are started.

Links to related issues and merge requests / references

n/a