Block insecure Dockerfiles with an agent security SKILL.md

Write SKILL.md for reviewing Dockerfile, docker-compose.yml, and related build scripts before merge. The agent produces merge-blocking findings, not generic security tips.

Repository context:
[paths to Dockerfiles, compose files, .dockerignore]
[base image policy: allowed registries, pin by digest or minor tag]
[runtime: Kubernetes, ECS, plain docker — affects healthcheck and signal handling]
[secret injection pattern: env at runtime, secrets mount, not build args for prod secrets]

Diff context (paste when using):
[PR diff or file paths changed]

SKILL.md must include:
1. Purpose — catch container misconfig before CI builds and pushes an image attackers can abuse.
2. When to use — any PR touching Dockerfile*, compose*, build workflows that docker build; when not to use — README-only.
3. Severity rubric the agent must use in findings:
   - Block: root USER 0, secrets in ENV/ARG/COPY layers, :latest-only base with no digest, missing .dockerignore exposing .git or .env, privileged: true or host mounts without justification.
   - Warn: no HEALTHCHECK when service is user-facing, running as root fallback, apt/apk upgrade in every build, shell as ENTRYPOINT for prod.
   - Note: style and layer caching — never block merge on style alone.
4. Required inputs: file list, intended environment (dev image vs prod image), and link to CI build job if any.
5. Review steps:
   - Read Dockerfile in build order; flag each RUN that embeds tokens, keys, or private URLs.
   - Confirm USER exists and is not root for prod-target images; if root required, demand comment with waiver id.
   - Check FROM pins: name registry, tag, and whether digest is required by policy.
   - Scan compose for secrets in environment: blocks, volume mounts of docker.sock, cap_add SYS_ADMIN.
   - Verify .dockerignore excludes .env*, **/credentials*, .git, and local build artifacts.
   - List findings as: [Block|Warn|Note] file:line — issue — suggested fix (one line).
6. PR comment template: Summary counts by severity, Findings table, Merge recommendation (block / approve with warns / approve).
7. Two example prompts: new Node service Dockerfile vs compose adding a sidecar.
8. Pitfalls: approving multi-stage builds that still COPY .env, confusing dev-only root with prod image, ignoring build-stage secrets that remain in layer history, waiving without ticket.
9. Done means: every Block resolved or waived with ticket, Warns acknowledged, no secret values reproduced in the comment.

Rules:
- Never echo real secret values from files; redact to [REDACTED].
- Do not invent CVEs; mention scanner output only if pasted.
- Output only SKILL.md markdown.