Decouple runtime image generation from pre-built SDKs

[Saipriya-1144]

Decouple runtime image builds from SDK publishing pipeline so runtime images can be generated independently.

Python: Introduced a multi-stage pythonSdkBuilder intermediate stage that builds Python from source using build.sh (the same script used by the SDK pipeline). The final image receives only the compiled /opt/python/ directory, eliminating storage account dependency and keeping the final image clean. Applied to both template.Dockerfile and noble.Dockerfile

Node: Introduced a multi-stage nodeDownloader intermediate stage that installs Node via n version manager. The final image receives only the Node binaries

Testing:

20260427.2 image is created with n, 20260427.3 is created with nvm

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