Dockerfile

A Dockerfile is the recipe of an image: a base image, then instructions that copy files, run commands and say how the container starts. docker build turns it into an image, with one layer for each instruction that changes files.

How it works

FROM picks the base, WORKDIR sets the folder, COPY brings files from the build context, RUN executes a command at build time, ENV and EXPOSE describe the configuration, USER sets who runs the program, and CMD or ENTRYPOINT set what runs. Instructions apply in order, from top to bottom.

What breaks

RUN runs at build time, CMD when the container starts: RUN node server.js hangs the build, since the server never exits. Each RUN also starts a fresh shell, so RUN cd /app has no effect on the next line; use WORKDIR.

In a Dockerfile

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

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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