.dockerignore and build speed

In plain English

When you run docker build, Docker first sends the build context, the folder around your Dockerfile, to the daemon before it builds anything. A .dockerignore file lists files and folders to leave out, such as node_modules and .git, so the context stays small and secrets stay out of the image. Docker caches each layer and reuses it only while its inputs are unchanged, so you copy dependency manifests before source code and keep expensive installs early. Because layers build from top to bottom, ordering matters: a change to source should not invalidate an expensive dependency install. A multi-stage build does the work in a first stage and copies only the finished result into a tiny final image.

Step by step

1. 1Create a .dockerignore that excludes node_modules, .git and build output.
2. 2Copy package*.json first, install dependencies, then copy the source.
3. 3Build once, then change only a source file and rebuild to see the cache hit.
4. 4Split the build into a build stage and a runtime stage with COPY --from.
5. 5Check the final image size with docker images.

FROM node:22-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:22-alpine
WORKDIR /app
ENV NODE_ENV=production
COPY --from=build /app/dist ./dist
USER node
EXPOSE 3000
CMD ["node", "dist/index.js"]

Common mistakes

• Copying all of the source before installing dependencies invalidates the install cache on every code edit.
• Sending the whole project as the build context makes every build slow and can leak files into the image.
• Keeping the compiler and dev dependencies in the final image bloats it unnecessarily.
• Running npm install instead of npm ci can skip the lockfile and produce unrepeatable builds.