Multimodal: Limit transformer version in Dockerfile

[faradawn]

What does this PR do ?

The current dockerfile does not contain certain packages like setuptools, which caused the build to fail on some devices.

More importantly, the transformer version is not below 5.0.0, which introduces breaking change.

This PR fixes the issue and has successfully performed the conversion of a llama model.

  [2026-02-17 00:38:01.616338] successfully saved checkpoint from iteration       1 to ./converted_mistral [ t 1/1, p 1/1 ]

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[janEbert]

/ok to test 50dcad4

[faradawn]

/ok to test 2318cfc

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22455400401

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22456323696