KAFKA-20448: Fix JVM Docker archive builds

[lh0156]

Summary

• Allow both JVM Docker stages to use a local Kafka archive when kafka_url is omitted.
• Capture the broker PID during CDS archive generation so shutdown does not depend on truncated Alpine BusyBox ps output.
• Preserve the existing URL download and signature verification path.

Tests

• RED: docker/docker_build_test.py with --image-type=jvm, --kafka-archive, and --build failed before this change with kafka_url: parameter not set.
• GREEN: the same local-archive JVM Docker build passed after this change, including the JSA generation stage.
• bash -n docker/jvm/jsa_launch
• git diff --check

Reviewers: Sean Quah squah@confluent.io

[squah-confluent]

Thanks for your PR. Please note that PR #22054 is already open for this issue.

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[lh0156]

Closing this duplicate as requested. Existing PR #22054 is already open for KAFKA-20448 and contains the same Docker JVM archive build change. No merge is claimed for this PR.