chore(docker): merge consecutive RUN instructions

[zarfishan20]

Fix Dockerfile syntax by ensuring BuildKit
Immediately after RUN instructions.

Note: The previous PR was closed unintentionally.

Resolves #3465

Checklist

• [x ] Required: I followed the contributing workflow
• [ x] Required: I verified that my code works as intended and resolves the issue as described
• Required: I ran make check-test locally: all warnings addressed, tests passed
• [ x] I used AI for code, documentation, tests, or communication related to this PR

[github-actions]

PR validation failed: No linked issue and no valid closing issue reference in PR description

[coderabbitai]

Caution

Review failed

The pull request is closed.

Summary by CodeRabbit

• Chores
  • Updated Docker build configurations for backend service images across multiple container variants to improve build efficiency.

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Walkthrough

The PR consolidates multiple consecutive RUN instructions across five backend Dockerfiles into fewer layers by merging APK setup, cache mounting, user creation, and chown operations, reducing image layers while maintaining functional behavior.

Changes

Cohort / File(s)	Change Summary
RUN instruction consolidation docker/backend/Dockerfile, docker/backend/Dockerfile.fuzz, docker/backend/Dockerfile.local, docker/backend/Dockerfile.test, docker/backend/Dockerfile.video	Merges separate RUN steps (APK setup, cache mounting, user creation, chown) into single RUN instructions; adjusts PIP_CACHE_DIR and APK_CACHE_DIR cache mount placement; removes redundant intermediate layers while preserving functionality.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~18 minutes

Possibly related PRs

• Add cache mounts to docker files #1452: Modifies backend Dockerfiles to consolidate RUNs and adjust cache-dir usage with related user/chown changes.
• Improve docker cache mounts #1495: Updates backend Dockerfile cache handling and RUN-step consolidation with integrated PIP/APK cache mounts.
• Apply rabbit suggestions on the e2e feature branch #3389: Updates CI configuration to use docker/backend/Dockerfile.fuzz path, affecting the same Dockerfile modified in this PR.

Suggested labels

backend, docker

Suggested reviewers

• arkid15r
• kasya

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud