container-image-hardening

Imported from securityskills/skills (container-security/docker/container-image-hardening/SKILL.md). Install upstream with npx skills add securityskills/skills --skill container-image-hardening. Copyright stays with the author.

Container Image Hardening

Review and fix Dockerfiles and images for production safety.

Dockerfile Review Checklist

• Base image: official, specific tag or digest, minimal variant (alpine, distroless, slim); never latest
• Multi-stage builds: build toolchains (compilers, package managers) excluded from final image
• Non-root user: USER directive with a dedicated UID; no sudo in image
• No secrets: no ENV with credentials, no COPY .env, no secrets baked into layers (they persist even if deleted later)
• Pinned dependencies: lockfiles used (npm ci, pip install -r requirements.txt with hashes)
• Healthchecks defined; ENTRYPOINT over CMD for enforced init
• Layer hygiene: combine apt operations and clean lists in one layer; .dockerignore covers .git, build artifacts

Scan and Gate

trivy image --severity HIGH,CRITICAL --exit-code 1 <image>
grype <image>
docker scout cves <image>

• Fail CI on critical CVEs with available fixes
• Track base image updates (renovate/dependabot for Dockerfiles)

Runtime Hardening

read_only: true
cap_drop: ["ALL"]
security_opt: ["no-new-privileges:true"]
tmpfs: [/tmp]

• Resource limits set (CPU/memory) to blunt DoS
• Root filesystem read-only; writable paths explicit tmpfs

Verification

• docker history <image> — no secret-looking layers
• dive <image> — image efficiency and wasted space
• Run as the image user: docker run --rm <image> id shows non-root

Output

Hardened Dockerfile, scan report before/after, and CI gate configuration.