fix: [3.0] avoid duplicate chown layers in runtime images

[zhikunyao]

Backport of #53174 to the 3.0 branch.

What

• create the milvus user and writable runtime directories before copying artifacts
• use Docker COPY --chown=milvus:milvus to avoid the duplicate recursive-chown image layer
• keep milvus:milvus as the default OSS runtime user while allowing downstream builds to set MILVUS_RUNTIME_USER
• generate the FIPS config as the milvus user

Verification

• git diff --check upstream/3.0..HEAD
• the net patch matches fix: avoid duplicate chown layers in runtime images #53174 exactly; stable patch ID: 4e7833bebecc661047e1e0b9552f319e8c24654d
• all six runtime Dockerfiles define the default runtime user and all 18 artifact copies use --chown=milvus:milvus
• all four FIPS Dockerfiles switch to milvus:milvus before openssl fipsinstall

Image build and multi-architecture validation are left to CI.

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[mergify]

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[INFO] PR Label Summary by Default
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• Target branch '3.0' requires a PR merged to master first
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• PR: fix: [3.0] avoid duplicate chown layers in runtime images #53251
• Title: fix: [3.0] avoid duplicate chown layers in runtime images
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[sre-ci-robot]

❌ CI Loop Results ac25e1b

Stage	Result	Duration	Tests
✅ Build	SUCCESS	16.6min	-
❌ Code-Check	FAILURE	8.7min	-
❌ UT-Integration	SKIPPED	-	-
❌ UT-GO	SKIPPED	-	-
✅ UT-CPP-Cov	SUCCESS	57.9min	8739 total, 8739 passed, 0 failed

Total: 80min | Pipeline | Artifacts

Failed Test Logs:

• Code-Check: view log