[image_spec]: Add git to default apt packages for pip/uv git URL installs

[amanchoudhri]

Why are the changes needed?

When building Docker images with ImageSpec, if the requirements contain git-based dependencies (e.g., git+https://github.com/...), the build fails because git is not installed in the base debian:bookworm-slim image. This causes uv pip sync or pip install to fail when trying to clone git repositories.

Example error scenario: Running a Flyte workflow with requirements that include:

flytekit @ git+https://github.com/exa-labs/flytekit.git@...

What changes were proposed in this pull request?

Always include git in the apt packages installed in the Docker image. This ensures pip/uv can install packages from git URLs without requiring users to manually add git to their apt_packages list.

Note: The NIX_DOCKER_FILE_TEMPLATE already includes git (line 170), so this brings the standard Docker template to parity.

How was this patch tested?

Manual verification that the generated Dockerfile now includes apt-get install git.

Human Review Checklist

• Verify adding git to all images is acceptable (adds ~50MB to image size)
• Consider if duplicate git entries (if user also specifies it) could cause issues (apt-get handles this gracefully)

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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Link to Devin run: https://app.devin.ai/sessions/e921c20129a54329820035d836c303e5
Requested by: Aman Choudhri (aman@exa.ai) (@amanchoudhri)

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