CI - Execute end to ends tests with Mock

Hello,

A colleague wants to use GitLab CI to execute end to end tests. it needs to run a server mock to avoid calling some microservices.

At the beginning, he used GitLab CI Services but the creation of a new docker image when a mock changed is a little bit heavy to use.

To unblock it, I recommended using, in his project, to run the mock server and tests in the same comand.

Do you have any recommendations for this use case ?