Dockerfile-spiekbriefje

A Dockerfile is the recipe for an image: each instruction runs in order and most of them add a layer. Layers are cached, so the order you write them in decides how fast your rebuilds are.

# syntax=docker/dockerfile:1
FROM node:22-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
CMD ["node", "server.js"]

💡 Start the file with # syntax=docker/dockerfile:1 to get the current BuildKit frontend — that is what enables heredocs, cache mounts and build secrets.

📋 Instruction Reference

🏗 Base Image & Metadata

# pin the base image — 'latest' makes builds irreproducible
FROM node:22-alpine

# name a stage so later stages can copy from it
FROM golang:1.23 AS build

# an ARG before the first FROM can parameterise the base image itself
ARG NODE_VERSION=22
FROM node:${NODE_VERSION}-alpine

# metadata; the OCI keys are the conventional ones
LABEL org.opencontainers.image.source="https://github.com/acme/app"
LABEL org.opencontainers.image.description="Acme API"

📂 Files & Working Directory

# always set an absolute working directory
WORKDIR /app

# copy from the build context
COPY package*.json ./
COPY . .

# copy and set ownership in one step, no extra layer for chown
COPY --chown=node:node . .

# copy with explicit permissions
COPY --chmod=755 entrypoint.sh /usr/local/bin/

# copy from another stage, or straight from another image
COPY --from=build /src/app /usr/local/bin/app
COPY --from=nginx:alpine /etc/nginx/nginx.conf /etc/nginx/nginx.conf

# ADD unpacks archives and fetches URLs — prefer COPY unless you need that
ADD archive.tar.gz /opt/
ADD https://example.com/file.tar.gz /tmp/

# declare a mount point for data that should not live in the image
VOLUME ["/data"]

⚙️ Build Steps

# shell form: runs through /bin/sh -c
RUN apt-get update && apt-get install -y --no-install-recommends curl \
    && rm -rf /var/lib/apt/lists/*

# exec form: no shell, so no globbing or variable expansion
RUN ["npm", "ci", "--omit=dev"]

# heredoc: a readable multi-line script (BuildKit)
RUN <<EOF
set -eux
apk add --no-cache curl
adduser -D appuser
EOF

# cache mount: keep the package cache between builds
RUN --mount=type=cache,target=/root/.npm npm ci

# secret mount: use a credential without baking it into a layer
RUN --mount=type=secret,id=npmrc,target=/root/.npmrc npm ci

# bind mount: read from the context without copying it in
RUN --mount=type=bind,source=package.json,target=package.json npm ci

🚀 Runtime Configuration

# environment variables are visible to the running container
ENV NODE_ENV=production
ENV PATH="/app/bin:$PATH"

# document the port — this does not publish it, -p does
EXPOSE 3000

# drop root as early as you can
RUN adduser -D appuser
USER appuser

# tell Docker how to check the app is alive
HEALTHCHECK --interval=30s --timeout=3s --start-period=20s --retries=3 \
  CMD wget -qO- http://localhost:3000/health || exit 1

# the signal that stops the process cleanly
STOPSIGNAL SIGTERM

# the command the container runs
CMD ["node", "server.js"]

⚔️ CMD vs ENTRYPOINT

# together: ENTRYPOINT is the binary, CMD holds the default arguments
ENTRYPOINT ["python", "app.py"]
CMD ["--port", "8000"]

# docker run image                 → python app.py --port 8000
# docker run image --port 9000     → python app.py --port 9000

⚠️ Prefer the exec form (["cmd", "arg"]). The shell form wraps your process in /bin/sh -c, which swallows SIGTERM and turns graceful shutdown into a ten-second kill.

🧱 Multi-Stage Builds

Several FROM instructions in one file: build with a fat toolchain, ship only the artifact.

# stage 1 — build
FROM node:22-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# stage 2 — the image that actually ships
FROM nginx:alpine
COPY --from=build /app/dist /usr/share/nginx/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

# a compiled binary on a near-empty base
FROM golang:1.23 AS build
WORKDIR /src
COPY . .
RUN CGO_ENABLED=0 go build -o /out/app ./cmd/app

FROM gcr.io/distroless/static-debian12
COPY --from=build /out/app /app
USER nonroot:nonroot
ENTRYPOINT ["/app"]

# build only up to a named stage — handy for a test or lint stage
docker build --target build -t app:build .

📁 .dockerignore

Everything in the build context is sent to the daemon and busts the cache. Exclude what the build does not need:

.git
node_modules
dist
*.log
.env
Dockerfile
.dockerignore

🧠 Best Practices

• Order by how often things change: base image, then dependency manifests, then npm ci/pip install, then the source. A source edit should not reinstall dependencies.
• Pin versions — node:22.11-alpine, not node:latest.
• One RUN per logical step, chaining installs and cleanup in the same layer (rm -rf /var/lib/apt/lists/*), or the deleted files still weigh on the image.
• Never bake secrets in. A deleted file stays in the layer below; use --mount=type=secret or build arguments that never reach the final stage.
• Run as a non-root user and keep the final stage as small as you can (alpine, distroless, scratch).
• Use COPY, not ADD, unless you specifically want URL fetching or archive extraction.
• Combine with --platform for multi-arch: docker buildx build --platform linux/amd64,linux/arm64.

🛠 Building & Running

# build from the Dockerfile in this directory
docker build -t my-app .

# build without the cache
docker build --no-cache -t my-app .

# a Dockerfile somewhere else
docker build -f docker/Dockerfile.prod -t my-app .

# pass a build argument
docker build --build-arg NODE_VERSION=20 -t my-app .

# pass a secret (never becomes a layer)
docker build --secret id=npmrc,src=$HOME/.npmrc -t my-app .

# build for another architecture
docker buildx build --platform linux/arm64 -t my-app .

# inspect the layers of the result
docker history my-app

# run it
docker run -p 3000:3000 my-app

📄 Full Example

# syntax=docker/dockerfile:1
FROM node:22-alpine AS deps
WORKDIR /app
COPY package*.json ./
RUN --mount=type=cache,target=/root/.npm npm ci

FROM node:22-alpine AS build
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY . .
RUN npm run build

FROM node:22-alpine
LABEL org.opencontainers.image.source="https://github.com/acme/app"
ENV NODE_ENV=production
WORKDIR /app
COPY --from=deps --chown=node:node /app/node_modules ./node_modules
COPY --from=build --chown=node:node /app/dist ./dist
USER node
EXPOSE 3000
HEALTHCHECK --interval=30s CMD wget -qO- http://localhost:3000/health || exit 1
CMD ["node", "dist/server.js"]

📚 Resources

• Dockerfile reference
• Building best practices
• BuildKit mounts (cache, secret, bind)
• Multi-stage builds