Lint Dockerfile Online — Hadolint-Style DL Rules

Lint Dockerfiles with syntax checks plus ~26 Hadolint DL best-practice rules. ShellCheck in RUN is not included.

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Dockerfile linter, Hadolint-style DL checks
Catch latest tags, ADD vs COPY, missing USER and other high-value issues before CI runs the full Hadolint CLI.

Developer notes

DL rules run on dockerfile-ast; syntax errors from dockerfile-utils are separate. ShellCheck (SC*) rules are intentionally out of scope v1. Info vs warning severities follow our DL mapping. Pin base images after fixing DL3006/DL3007 warnings.

When teams pick this route

Quick lint on a Dockerfile pasted in a review comment. Educate juniors with official DL codes and wiki links. Pre-flight check before pushing to a registry pipeline. Compare against Hadolint CLI on small files.

Worked examples

Latest tag + ADD + apt without pins

Dockerfile input

FROM ubuntu:latest MAINTAINER [email protected] RUN apt-get update && apt-get install curl RUN echo "starting build" ADD app.tar.gz /app/ WORKDIR relative EXPOSE 80 CMD echo start

Lint highlights

Warnings: DL3007 (latest), DL4000 (MAINTAINER), DL3020 (ADD), DL3008/DL3014/DL3015 (apt), DL3000 (relative WORKDIR), DL4001 (no USER), DL3025 (shell CMD).

Improved production-style fragment

Dockerfile input

FROM ubuntu:22.04 RUN apt-get update \ && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends curl=7.81.0-1 \ && rm -rf /var/lib/apt/lists/* COPY --chown=app:app src /app/ USER app WORKDIR /app CMD ["./server"]

Lint highlights

Fewer DL warnings: pinned apt, non-root USER, JSON CMD, COPY instead of ADD.

Navigate related DevOps tools

Format first with format Dockerfile. Compose stacks: validate Docker Compose.

Dockerfile linter FAQ

DL rules without ShellCheck

Is this full Hadolint?

No. It applies about 26 DL rules plus syntax checks. ShellCheck inside RUN is not included in v1.

Why DL codes?

They match the official Hadolint wiki so you can compare with CI and learn remediation.

Errors vs warnings?

Syntax issues from dockerfile-utils are errors. DL rules are mostly warnings or info.

DevOps & Infra

Terraform HCL format/validate/minify, Dockerfile format & lint, Docker Compose and .env validation.

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