The frontend image kept shipping vulnerable OS packages (nginx 1.28.3-r1,
curl/libcurl 8.19.0, c-ares 1.34.6) despite the apk upgrade line, for two
independent reasons:
1. The runtime stage's apk upgrade layer was cached indefinitely — the
CACHEBUST build-arg CI passes (github.run_number) was only declared in
the builder stage, and ARGs don't cross stage boundaries. Both
Dockerfiles now redeclare CACHEBUST in the runtime stage and consume it
in the apk RUN, so every build re-runs the upgrade and picks up current
Alpine security updates.
2. nginx itself can never upgrade via apk on the nginx.org-based image:
the bundled nginx-module-* packages pin the exact nginx version, so
Alpine's patched 1.28.3-r4 is unreachable (verified empirically —
apk add --upgrade nginx is a silent no-op). nginx fixes must come via
the base tag, so bump to nginx:1.30-alpine (current stable, 1.30.4 on
Alpine 3.24, same nginx.org conf.d layout — drop-in).
Verified: local image build scans clean with Trivy (0 OS findings, was 21);
container serves /health, SPA fallback, and BRAND_TITLE envsubst as non-root
nginx user.
Closes code-scanning alerts 371-374, 376-392 (nginx HTTP/2 & module CVEs,
curl CVE-2026-5773/-6276 + 6 medium, c-ares CVE-2026-33630).
Paul Nothaft
ae98e7ad74