Dockerfile keywords (Also known as Instructions)

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Dockerfile keywords

Dockerfiles use specific keywords (also called instructions) to define how a container image is built. Here are the most important ones:

Basic Instructions

• FROM – Specifies the base image (e.g., FROM ubuntu:20.04).
• RUN – Executes commands during the build (e.g., RUN apt-get update).
• CMD – Specifies the default command to run in a container (e.g., CMD ["nginx", "-g", "daemon off;"]).
• ENTRYPOINT – Defines the main executable of the container (e.g., ENTRYPOINT ["python", "app.py"]).
• WORKDIR – Sets the working directory inside the container (e.g., WORKDIR /app).
• COPY – Copies files from the host to the image (e.g., COPY . /app).
• ADD – Similar to COPY, but can also extract tar archives and handle URLs.
• EXPOSE – Declares the port the container will listen on (e.g., EXPOSE 80).

Optimization & Efficiency

• LABEL – Adds metadata to an image (e.g., LABEL maintainer="twtechs671@gmail.com").
• ENV – Sets environment variables (e.g., ENV NODE_ENV=production).
• ARG – Defines build-time variables (e.g., ARG VERSION=1.0).
• VOLUME – Declares a volume for persistent storage (e.g., VOLUME /twtech-data).

User & Execution Control

• USER – Specifies the user to run the container as (e.g., USER twtech-root).
• ONBUILD – Triggers commands when the image is used as a base image.

Best Practices

• Use multi-stage builds to reduce image size (FROM builder AS base).
• Minimize layers by chaining commands (RUN apt-get update && apt-get install -y curl).
• Use .dockerignore to exclude unnecessary files.