fix-dockerfile

A checker and fixer for an existing Dockerfile, the recipe used to build an application container. It examines security, speed, and common setup rules, then produces a corrected version.

In plain words
What is it for?
Use it to review a Dockerfile for production or development, record issues by severity, apply the required corrections, and note platform-specific next steps.
Why use it?
It identifies problems before the container is built or published, including issues that could make the image less secure or efficient.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/azure/containerization-assist/fix-dockerfile
Any agent
npx skills add Azure/containerization-assist --skill fix-dockerfile
Clone the repo
git clone --depth 1 https://github.com/Azure/containerization-assist

Made for: Claude Code, Codex.

Per session 95 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,477 The whole file, excluding the scripts and references it only reads on demand.
Security scan B 3 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00095 $0.03477
Opus 5 $0.00048 $0.01739
Sonnet 5 $0.00019 $0.00695
Haiku 4.5 $0.00010 $0.00348

Measured 3d ago against content hash da8980890aaa, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade B, and why

fix-dockerfile scanned grade B with 3 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Asks for rootlowPrivilege escalation

A mod that escalates privileges can change anything on the machine, not only the project.

| `avoid-sudo` | Drop `sudo ` prefix. If the action requires root, move it before the final `USER` directive. |

Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.

Recursive force deletemediumDestructive command

rm -rf with a variable or a broad path is one typo away from removing the wrong tree.

| `optimize-package-install` | `RUN` line with `apt-get install` but NO `rm -rf /var/lib/apt/lists/*` in the same `RUN` | warning | Combine update + install + cleanup in one `RUN`. |

Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

| `require-healthcheck` | Add a HEALTHCHECK matching the base image's available tools: `curl -fsS http://localhost:<port>/health \|\| exit 1` if curl is present; `wget --spider -q http://localhost:<port>/health \|\| exit
skills/fix-dockerfile/SKILL.md · 229 lines

How it starts

The opening of the file, as written. The whole thing — 229 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Fix Dockerfile

Validate an existing Dockerfile against a fixed rule set, score it, and produce a fully remediated version. Deterministic — every rule below is a regex or a structural check, not a judgment call.

Inputs

Field Required Description
dockerfile or path yes Either the file contents OR an absolute path. If path is given, read it.
environment optional production (default) or development. Some rules downgrade to suggestions in dev.
targetPlatform optional linux/amd64 (default) or linux/arm64. Used only in the Next steps section.

If neither is provided → ask the user for the Dockerfile path. STOP.

Procedure

Step 1 — Read the file

If path is given, read the file content. Reject if it doesn't contain at least one FROM line (case-insensitive) — that's not a Dockerfile.

Step 2 — Run every rule

Apply each rule below in order. For each match, record an issue with { ruleId, category, severity, line, message }.

severityerror (must fix) | warning (should fix) | info (nice to fix). categorysecurity | performance | bestPractices.

When a rule says "scan lines", split the file on \n, trim each line, ignore empty lines and lines starting with #.

Security rules (highest priority)
ruleId Pattern Severity Message
block-root-user Any line matches ^USER\s+(root|0)\s*$ (case-insensitive multiline) error Running as root is forbidden. Replace with a non-root user.
no-root-user No USER line at all, OR final USER line argument is root/0 error Container must declare a non-root USER instruction.
block-secrets-in-env Any ENV or ARG line matches (?i)(PASSWORD|PASSWD|SECRET|TOKEN|API[_-]?KEY|PRIVATE[_-]?KEY|ACCESS[_-]?KEY|CREDENTIAL|DATABASE_URL|CONNECTION_STRING|REDIS_URL|MONGODB?_URI|_DSN)\b.*=\s*\S+ with a non-empty, non-placeholder value error Hardcoded secret in ENV/ARG. Inject via runtime (Kubernetes Secret).
no-secrets Same as above but covers RUN export FOO=… and RUN echo "FOO=…" shapes error Hardcoded secret in RUN.
avoid-sudo Any line matches (?i)\bsudo\b warning Drop sudo; run the entire stage as the required user instead.
avoid-apt-upgrade Any line matches (?i)apt-get\s+(upgrade|dist-upgrade) warning Avoid apt-get upgrade; pin package versions.

Read the full file on GitHub · 229 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 3d ago First seen · 229 lines · 95 tokens per session scan B da8980890aaa

Subscribe to this mod's changes

fix-dockerfile is a skill published in the GitHub repository Azure/containerization-assist (41 stars, last pushed 13d ago), licensed MIT. It adds 95 tokens to every session and 3,477 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it B with 3 findings (asks for root, recursive force delete, makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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