Dockerfile Best Practices: Tips for Efficient Image Building

Dockerfiles are the configuration files used to build Docker images; while they are easy to write, many subtle choices affect image size, build speed, and maintainability. This guide outlines detailed tips to achieve best‑practice Dockerfile authoring.

1. Pay attention to build order

FROM debian
# COPY . ./app   (removed in original example)
RUN apt-get update
RUN apt-get -y install cron vim ssh
COPY . ./app

Placing frequently‑changing steps (like copying source code) later preserves earlier layer caches, reducing rebuild time, especially in development.

2. Use more precise COPY

Only copy files required for the image; exclude unnecessary items such as node_modules via a .dockerignore file.

3. Merge commands

# Bad practice (two RUN layers)
RUN apt-get update
RUN apt-get -y install cron vim ssh

# Better practice (single RUN layer)
RUN apt-get update && apt-get -y install cron vim ssh

Combining related commands into one RUN reduces the number of layers and improves cache reuse.

4. Remove unnecessary dependencies

RUN apt-get update && apt-get -y install --no-install-recommends cron

Install only what is required; avoid installing debug tools or recommended packages unless explicitly needed.

5. Clean package manager caches

RUN apt-get update && apt-get -y install --no-install-recommends cron \
    && rm -rf /var/lib/apt/lists/*

Deleting the apt cache after installation keeps the final image size small.

6. Use official base images

When a language runtime is needed (e.g., Node.js), pull the official image instead of building it from a generic Linux base.

7. Use clear tags

Avoid FROM node:latest; specify an exact version tag to guarantee reproducibility.

8. Choose appropriately sized base images

Even with the same Node version, different base distributions can vary dramatically in size; prefer minimal images when possible.

9. Build from source in a consistent environment

Ensure the build environment matches the target runtime to avoid incompatibilities.

10. Install application dependencies separately

System packages and application‑level dependencies should be installed in distinct steps.

11. Multi‑stage builds

FROM golang:1.12 AS builder
WORKDIR /app
COPY ./ ./
RUN go build -o myapp test.go

FROM debian:stable-slim
COPY --from=builder /app/myapp / 
CMD ["./myapp"]

Multi‑stage builds allow you to compile with a full toolchain in the first stage and copy only the final artifact into a minimal runtime image, dramatically reducing final image size (requires Docker 17.05+).

By applying these tips appropriately to your own projects, you can produce Docker images that are lean, fast to build, and easier to maintain.