Dockerfile Instructions Explained: FROM, RUN, COPY, CMD, ENTRYPOINT and More

Dockerfile Instructions Explained#

A Dockerfile is the build recipe for a Docker image. This guide explains the most important Dockerfile instructions with examples, syntax, common mistakes, and best practices.

Quick Dockerfile example#

FROM node:20-alpine WORKDIR /app COPY package*.json ./ RUN npm ci --omit=dev COPY . . EXPOSE 3000 CMD ["node", "server.js"]

FROM#

FROM selects the base image. Every Dockerfile normally starts with a base image.

FROM nginx:alpine FROM node:20-alpine FROM python:3.12-slim

WORKDIR#

WORKDIR sets the working directory inside the image.

WORKDIR /app

COPY and ADD#

COPY copies files from your build context into the image. Prefer COPY for normal file copying.

COPY package*.json ./ COPY src/ ./src/

RUN#

RUN executes commands during image build. It is used for installing packages and preparing the image.

RUN npm ci --omit=dev RUN apt-get update && apt-get install -y curl

EXPOSE#

EXPOSE documents which port the containerized app listens on. It does not publish the port by itself.

EXPOSE 3000

CMD#

CMD defines the default command when a container starts.

CMD ["node", "server.js"]

ENTRYPOINT#

ENTRYPOINT defines the main executable. It is often used when the container behaves like a command-line tool.

ENTRYPOINT ["nginx", "-g", "daemon off;"]

ENV and ARG#

ARG is available at build time. ENV is available in the image and container environment.

ARG APP_VERSION=dev ENV NODE_ENV=production

USER#

USER runs the container process as a non-root user when configured correctly.

USER node

HEALTHCHECK#

HEALTHCHECK tells Docker how to test if a container is healthy.

HEALTHCHECK --interval=30s --timeout=5s CMD curl -f || exit 1

.dockerignore#

A .dockerignore file prevents unnecessary files from being sent to the Docker build context.

node_modules .git .env coverage Dockerfile*

Common mistakes#

• Copying the entire project before installing dependencies, causing slow rebuilds.
• Committing secrets into images.
• Using very large base images without reason.
• Running everything as root.
• Forgetting .dockerignore .

Best practices#

• Use small official base images.
• Pin important versions where possible.
• Copy dependency files first for better build cache.
• Use non-root users for app containers where practical.
• Keep one main process per container.
• Use multi-stage builds for compiled apps.

Related Melochip guides#

• Docker Learning Path
• Docker Images vs Containers Explained
• GitLab Docker Build Pipeline

Official references#

• Dockerfile reference