Dockerfile Validator

Dockerfile Validator

Validate Dockerfiles with deterministic stages, clear severity reporting, and explicit fallbacks when tools or network access are constrained.

Trigger Phrases

Use this skill when the user asks for tasks like:

• "validate this Dockerfile"
• "lint/check my Dockerfile"
• "security scan Dockerfile"
• "optimize Docker image size/build time"
• "review Dockerfile before merge"
• "find issues in Dockerfile.prod/Dockerfile.dev"

Use / Do Not Use

Use this skill for:

• Syntax and lint validation
• Security and secrets checks
• Best-practice and performance review
• Dockerfile hardening before CI/CD or production

Do not use this skill for:

• Generating a new Dockerfile from scratch (use dockerfile-generator)
• Running containers, debugging runtime behavior, or image registry operations

Local Files In This Skill

• Validator script: scripts/dockerfile-validate.sh
• References:
  • references/security_checklist.md
  • references/optimization_guide.md
  • references/docker_best_practices.md
• Example Dockerfiles: examples/*.Dockerfile

Deterministic Execution Flow (Required)

Run these steps in order. Do not skip steps unless a documented fallback branch applies.

1. Preflight and Path Setup

Assume repo root as working directory:

cd /path/to/repo
SKILL_DIR="devops-skills-plugin/skills/dockerfile-validator"
TARGET_DOCKERFILE="Dockerfile"   # replace when user provides a path

Validate inputs before running tools:

test -f "$SKILL_DIR/scripts/dockerfile-validate.sh"
test -f "$TARGET_DOCKERFILE"

If either check fails, stop and report the exact missing path.

2. Read the Target Dockerfile Explicitly

Use explicit file-read commands (not abstract "Read tool" wording):

sed -n '1,220p' "$TARGET_DOCKERFILE"

If needed for long files:

sed -n '220,440p' "$TARGET_DOCKERFILE"

3. Run Validation Script

Primary command:

bash "$SKILL_DIR/scripts/dockerfile-validate.sh" "$TARGET_DOCKERFILE"

Optional captured run for structured reporting:

bash "$SKILL_DIR/scripts/dockerfile-validate.sh" "$TARGET_DOCKERFILE" | tee /tmp/dockerfile-validator.out

4. Classify Findings by Severity (Standard)

Use this standard severity model:

• Critical
  • Hardcoded secrets/credentials
  • Explicit root runtime with high-risk context
  • High-impact security policy failures
• High
  • Checkov failures for container hardening
  • hadolint errors likely to cause insecure/unreliable builds
  • Missing or unsafe runtime-user posture (USER)
• Medium
  • :latest image tags, missing pinning, cache-cleanup misses
  • Build cache inefficiency and layered install anti-patterns
• Low
  • Style/info guidance and non-blocking optimization suggestions

5. No-Issue Fast Path (Required)

If validation has no actionable findings:

• Return a concise pass summary.
• Do not open reference files.
• Do not generate fix diffs.

Use fast path when all are true:

• Script reports overall pass.
• No security failures.
• No error/warning findings requiring user action.

6. Reference Loading Rules (Only When Findings Exist)

Only read references that match actual findings. Read each required file once.

Issue-to-reference mapping:

Explicit read commands:

sed -n '1,220p' "$SKILL_DIR/references/security_checklist.md"
sed -n '1,220p' "$SKILL_DIR/references/optimization_guide.md"
sed -n '1,220p' "$SKILL_DIR/references/docker_best_practices.md"

For targeted extraction:

rg -n "USER|secrets|EXPOSE|HEALTHCHECK" "$SKILL_DIR/references/security_checklist.md"
rg -n "multi-stage|cache|layer|dockerignore" "$SKILL_DIR/references/optimization_guide.md"
rg -n "FROM|COPY|ADD|WORKDIR|CMD|ENTRYPOINT|latest" "$SKILL_DIR/references/docker_best_practices.md"

7. Produce Standard Report Output

Use this template for every non-fast-path run:

## Dockerfile Validation Report
- Target: <path>
- Command: `bash <skill-script> <target>`
- Overall result: PASS | FAIL | PARTIAL (fallback)

### Critical
- <issue or `None`>

### High
- <issue or `None`>

### Medium
- <issue or `None`>

### Low
- <issue or `None`>

### Recommended Fixes
- <specific code-level fix per actionable issue>

### References Used
- <list only files actually read>

### Fallbacks Used
- `None` or exact fallback branch + reason

8. Offer Fix Application

After reporting:

• Ask whether to apply fixes.
• If user approves, patch the Dockerfile and rerun validation.

Fallback Behavior (Explicit)

When the primary script cannot complete, use deterministic fallback branches and report them.

Fallback A: Python/Tool Install Constraint

Condition:

• Script exits with tool-install failure (for example Python missing, package install blocked, or restricted environment).

Action:

1. Report primary failure and why.
2. Run manual minimum checks:

# Basic syntax signal (if Docker is available)
DOCKERFILE_DIR="$(dirname "$TARGET_DOCKERFILE")"
docker build --no-cache -f "$TARGET_DOCKERFILE" "$DOCKERFILE_DIR"

# High-value static checks
grep -nEi "^[[:space:]]*FROM[[:space:]]+.*:latest" "$TARGET_DOCKERFILE" || true
grep -nEi "^[[:space:]]*(ENV|ARG)[[:space:]].*(password|secret|token|api[_-]?key)[[:space:]]*=" "$TARGET_DOCKERFILE" || true
grep -nEi "^[[:space:]]*USER[[:space:]]+(root|0(:0)?)$" "$TARGET_DOCKERFILE" || true
grep -nEi "^[[:space:]]*HEALTHCHECK[[:space:]]+" "$TARGET_DOCKERFILE" || true

3. Classify output with PARTIAL result and clearly label skipped checks.

Fallback B: hadolint Not Available but Docker Available

Use hadolint container image:

docker run --rm -i hadolint/hadolint < "$TARGET_DOCKERFILE"

Fallback C: No Docker, No hadolint/checkov

Run only manual regex-based checks (Fallback A step 2), clearly mark as PARTIAL, and state which scanners were skipped.

Quick Command Set

Validate one Dockerfile

cd /path/to/repo
bash devops-skills-plugin/skills/dockerfile-validator/scripts/dockerfile-validate.sh Dockerfile

Validate alternate file

cd /path/to/repo
bash devops-skills-plugin/skills/dockerfile-validator/scripts/dockerfile-validate.sh Dockerfile.prod

Validate skill examples

cd /path/to/repo/devops-skills-plugin/skills/dockerfile-validator
bash scripts/dockerfile-validate.sh examples/good-example.Dockerfile
bash scripts/dockerfile-validate.sh examples/security-issues.Dockerfile

Run regression checks (CI entrypoint)

cd /path/to/repo
bash devops-skills-plugin/skills/dockerfile-validator/scripts/test_validate.sh

Optional strict mode for CI environments that must enforce ShellCheck:

STRICT_SHELLCHECK=true bash devops-skills-plugin/skills/dockerfile-validator/scripts/test_validate.sh

Progressive Disclosure Rules

• Always read the target Dockerfile first.
• Do not read any reference files unless findings require them.
• Read only the matching reference file(s) from the issue-to-reference mapping.
• Do not reread the same reference unless new issue categories appear.

Done Criteria

Consider this skill execution complete only when all conditions below are satisfied:

• Trigger matched a Dockerfile validation/lint/security/optimization request.
• Target Dockerfile path was explicitly verified.
• Validation command (or explicit fallback) was executed.
• Findings were reported using severity buckets (Critical, High, Medium, Low).
• Reference usage matched issue categories and was explicitly listed.
• No-issue fast path skipped unnecessary reference reads.
• If fixes were applied, validation was rerun and final status reported.

Resources

• Script: scripts/dockerfile-validate.sh
• CI/regression entrypoint: scripts/test_validate.sh
• Security reference: references/security_checklist.md
• Optimization reference: references/optimization_guide.md
• Best-practices reference: references/docker_best_practices.md
• Examples: examples/good-example.Dockerfile, examples/bad-example.Dockerfile, examples/security-issues.Dockerfile, examples/python-optimized.Dockerfile, examples/golang-distroless.Dockerfile

Source Links

• Docker Build Best Practices
• Dockerfile Reference
• Checkov Dockerfile Scanning
• hadolint

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