Document temporary AIPCC index workaround in Dockerfile

[coderabbitai]

Context

This issue tracks the suggestion from PR #2678 to add documentation about the use of --extra-index-url https://pypi.org/simple as a temporary workaround for gaps in the AIPCC index.

Description

The --extra-index-url https://pypi.org/simple pattern appears consistently across Power-specific builds (ppc64le/s390x) in jupyter/datascience/ubi9-python-3.12/Dockerfile.cpu to compensate for missing packages in the AIPCC index (e.g., cython, numpy>=1.22, protobuf>=4.25.1, cryptography dependencies).

Requested Action

Add a top-level comment in the Dockerfile (near the first occurrence or in the file header) documenting that:

• This is a temporary mitigation for AIPCC index limitations
• It should be revisited if/when the AIPCC index coverage improves
• List the known missing packages that necessitate this workaround

Affected Lines

The pattern appears at multiple locations:

• Lines 189-190 (pyarrow builder)
• Lines 241-246 (onnx builder)
• Lines 347-348 (pyarrow wheel installation)
• Lines 361-363 (onnx wheel installation)
• Line 428 (pylock.toml installation)

References

• PR: RHAIENG-948: fix(jupyter datascience on IBM/Power): build failure due to AIPCC base (missing curl and openssl -devel packages) and AIPCC wheels (some prereqs are missing) #2678
• Review comment: RHAIENG-948: fix(jupyter datascience on IBM/Power): build failure due to AIPCC base (missing curl and openssl -devel packages) and AIPCC wheels (some prereqs are missing) #2678 (comment)
• Requested by: @jiridanek