fix(docker): run containers as non-root user and update alpine to 3.21

[rohilsurana]

Summary

• Both Dockerfile and Dockerfile.dev ran as root (no USER directive)
• Added non-root frontier user and USER frontier directive to both
• Updated base image from alpine:3.18 (June 2023, approaching EOL) to alpine:3.21

Why this is safe

• Frontier binds to ports 8002, 9000, 8083 — all >1024, no root needed
• Zero file writes at runtime — no os.Create/os.Write/os.Mkdir in app code
• Binary at /usr/bin/frontier only needs read+execute, not ownership
• Config files are read-only at runtime

Test plan

• Build production image: docker build -t frontier-test .
• Build dev image: docker build -f Dockerfile.dev -t frontier-dev-test .
• Verify process runs as non-root: docker exec <container> whoami → frontier
• Verify all ports (8002, 9000, 8083) respond normally
• Verify mounted config files are readable by the frontier user

[vercel]

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Project	Deployment	Actions	Updated (UTC)
frontier	Ready	Preview, Comment	Jun 3, 2026 5:15pm

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📥 Commits

Reviewing files that changed from the base of the PR and between 2e93a90 and 71f0714.

📒 Files selected for processing (2)

• Dockerfile
• Dockerfile.dev

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[coveralls]

Coverage Report for CI Build 26900932352

Coverage remained the same at 43.138%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	38013
Covered Lines:	16398
Line Coverage:	43.14%
Coverage Strength:	12.08 hits per line

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💛 - Coveralls