Unity build fails to access private Git repositories in Docker environment

**Orb version**

1.6.0

**Execution environment**

- Executor: unity/windows-2019
- Target platform: windows-il2cpp
- Unity Editor version: 2021.3.24f1

**Bug description**

The `unity/build` command fails to access private Git repositories due to lack of Git credential configuration in the Docker environment. This command works fine in Linux and MacOS execution environments.

**How to reproduce**

- Set up a Unity project with dependencies on a private Git repository
- Configure CircleCI to build the project using `game-ci/unity` orb
- Observe failure in resolving private repositories during the build

**Expected behavior**

The build process should authenticate and access private Git repositories seamlessly, resolving all dependencies.

**Additional details**

Returned error:
```
An error occurred while resolving packages:
Project has invalid dependencies:
: Error when executing git command. Host key verification failed.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```
Currently, the script doesn't handle Git authentication for private repositories inside the Docker container, leading to failed builds when private dependencies are involved.