Fix Docker building instructions

[janEbert]

What does this PR do ?

It's never mentioned that the Megatron-LM submodule has to be initialized, even though it's used in the Dockerfile. So these instructions are added.

Changelog

• Add submodule initialization to Docker build instructions.

Additional Information

• Related to uv/pip installation instructions #1168

Summary by CodeRabbit

• Documentation
  • Updated contribution guide with setup instructions for third-party submodules.
  • Added clarified Docker build command instructions for contributors.

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request adds setup instructions for Git submodules and includes a concrete Docker build command in CONTRIBUTING.md, specifying the use of docker/Dockerfile.ci with the megatron-bridge image tag.

Changes

Cohort / File(s)	Summary
Documentation Updates CONTRIBUTING.md	Added Git submodule initialization step (git submodule update --init --recursive) and a concrete docker build command using -f docker/Dockerfile.ci -t megatron-bridge.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• build: Add setup.py to first layers #2421: Modifies docker/Dockerfile.ci to include Megatron-LM/setup.py, which is directly referenced by the Docker build command added in this PR.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding Docker build instructions, which directly corresponds to the PR's objective of documenting the submodule initialization step needed for Docker builds.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Test Results For Major Changes	✅ Passed	This PR contains only a minor documentation update to CONTRIBUTING.md that clarifies Git submodule initialization, with zero code changes and 8 lines added.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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