Dockerfile basics (build an image)

Topic: Containers core

Write a Dockerfile with FROM, RUN, COPY, and CMD to build a container image. Use multi-stage builds to keep the final image small. Use this when creating a custom image for your application or when optimizing build time and image size.

Intent: How-to

Quick answer

  • FROM base image (e.g. alpine, debian); RUN for commands (apt install, pip install); COPY to add files; CMD or ENTRYPOINT for the default command. Each instruction is a layer; order for cache (least-changing first).
  • Multi-stage: FROM builder AS build; RUN build steps; FROM runtime; COPY --from=build /app /app. Final image has only runtime and built artifact, not build tools. Use .dockerignore to exclude files from context.
  • Build: docker build -t myimage:tag . Run the container and test; push to a registry with docker push. Prefer specific base tags (alpine:3.18) not latest; pin dependency versions in RUN.

Prerequisites

Steps

  1. Minimal Dockerfile

    FROM alpine:3.18 RUN apk add --no-cache nginx COPY nginx.conf /etc/nginx/ CMD ["nginx", "-g", "daemon off;"] Build: docker build -t mynginx .

  2. Layer order and cache

    Put COPY of dependency files (package.json, requirements.txt) before COPY of source; then RUN install. So install layer is cached until deps change. Put COPY of source last.

  3. Multi-stage (optional)

    First stage: FROM golang AS build; COPY . .; RUN go build. Second stage: FROM alpine; COPY --from=build /app/binary .; CMD ["./binary"]. Final image has no Go toolchain.

  4. Build and test

    docker build -t myapp:1.0 .; docker run --rm myapp:1.0. Add .dockerignore (git, docs, test) to speed build. Push: docker tag myapp:1.0 registry/myapp:1.0; docker push registry/myapp:1.0.

Summary

Write a Dockerfile with FROM, RUN, COPY, CMD; use multi-stage to reduce size; build with docker build and test. Use this to create and maintain application images.

Prerequisites

Steps

Step 1: Minimal Dockerfile

Create a Dockerfile with FROM, RUN, COPY, and CMD; build with docker build -t name .

Step 2: Layer order and cache

Order instructions so dependency install is cached; copy source last.

Step 3: Multi-stage (optional)

Use multi-stage to build in one stage and copy artifacts into a smaller runtime image.

Step 4: Build and test

Build, run, and test; use .dockerignore; tag and push to a registry.

Verification

  • Image builds; container runs and behaves as expected; image size is acceptable.

Troubleshooting

Build fails — Check RUN commands (paths, packages); ensure context has required files. Image too large — Use multi-stage; use smaller base (alpine); remove cache in same layer (apt clean, rm -rf /var/cache).

Next steps

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