Run integration tests in docker with specific git version

I want our integration tests (only the new ones, we can ignore the deprecated ones) to run in a docker container using a specific git version.

Benefits:
* failures will be more easily reproduced by running the container locally
* we'll be able to set the git version to the minimum git version that Lazygit supports, meaning we won't accidentally ship features that break on our minimum supported git version.

Considerations:
* There may be no downsides to this: I assume github actions already uses docker under the hood, so startup time should be the same.
* We'll need to ensure we're able to cache the docker image