A Dockerfile is a plain-text recipe for building a Docker image. You write it once in your project; Docker follows its instructions to create the package.

For a Node app, it normally says: start from a Node base image, copy the project in, install dependencies, and say how to start the app.

Basic Node example

FROM node:22-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

After you save this as Dockerfile, the next note shows how to turn it into an image with docker build.

FROM

FROM chooses the base image.

Examples:

• node:22-slim
• python:3.12-slim
• nginx:alpine

Choose a base image that matches the runtime and security needs.

WORKDIR

WORKDIR sets the directory for later instructions.

It is better than repeatedly writing absolute paths.

COPY

COPY moves files from the build context into the image.

Copy dependency files first, install dependencies, then copy the rest of the source. This improves build cache usage.

RUN

RUN executes commands during image build.

Examples:

• Install dependencies.
• Build application artifacts.
• Compile assets.

Every RUN contributes to image layers.

EXPOSE

EXPOSE documents which port the container expects to listen on.

It does not publish the port by itself. You still need -p or platform configuration.

CMD

CMD defines the default command when a container starts.

There should usually be one main foreground process.

Quick revision

• Dockerfile builds an image.
• FROM chooses the base.
• COPY brings files into the image.
• RUN executes build-time commands.
• CMD is the default runtime command.