Update docs/source/en/custom_container.md

[alvarobartt]

What does this PR do?

This PR updates the docs/source/en/custom_container.md guide on how to build a custom TEI container with Docker, to include the git submodule update --init to include the required submodules for the CUDA images.

This PR is a follow up of #567

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@Narsil

[HuggingFaceDocBuilderDev]

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