Dockerfile Instructions Cheat Sheet: Every Keyword Explained

Every Dockerfile instruction with the gotchas that trip people up.

Build instructions, runtime config, and multi-stage patterns.

Instructions

Instruction	Purpose
FROM img AS stage	Base image / named stage
RUN cmd	Execute at build time (new layer)
COPY src dst	Copy files (preferred over ADD)
ADD url/tar dst	COPY + auto-extract / fetch
WORKDIR /app	Set working dir
ENV K=V	Persistent env var
ARG K	Build-time variable
EXPOSE 8080	Document port (no publish)
USER node	Drop root
VOLUME /data	Declare mount point
HEALTHCHECK	Container liveness probe

ENTRYPOINT vs CMD

Form	Behavior
ENTRYPOINT ["app"]	Fixed executable
CMD ["--flag"]	Default args (overridable)
CMD ["app", "--flag"]	Default command, fully overridable

Multi-stage build

Dockerfile

FROM node:20 AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM nginx:alpine
COPY --from=build /app/dist /usr/share/nginx/html

Key takeaways

Prefer COPY over ADD unless you need auto-extract or remote fetch.
ENTRYPOINT sets the fixed binary; CMD supplies overridable defaults.
Order layers least-to-most volatile to maximize cache hits.

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