Update code-change-guide add back build SDK container

[Abacn]

Previously removed in #34640. But to test changes in sdks/java/container (e.g. boot.go and Dockerfile), one still needs to build custom SDK container

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[gemini-code-assist]

Summary of Changes

Hello @Abacn, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the code-change-guide.md documentation to reintroduce essential instructions for building and using custom Java SDK containers. This change is crucial for developers who need to test modifications within the sdks/java/container module, as the ability to build custom containers was previously removed but is necessary for a complete testing workflow. The updated guide now provides clear steps for this process, ensuring developers can effectively validate their changes.

Highlights

• Documentation Update: Reinstated comprehensive instructions within the code-change-guide.md documentation for building and utilizing custom Java SDK containers.
• Developer Workflow Improvement: Enabled developers to effectively test changes made to sdks/java/container by providing clear steps for building and deploying custom SDK images, addressing a previously identified gap in the workflow.

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