Introduce a Gemfile dedicated to Overcommit to avoid conflicts

What does this MR do?

This introduces a Gemfile dedicated to Overcommit to avoid conflicts

Running Overcommit outside of the context of a Gemfile can lead to conflicts when requiring gems. Using a dedicated Gemfile instead of the default one avoids loading all the app's gems before running Overcommit.

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content