Dockerfile Language Guide

This document provides an overview of common keywords and instructions in Dockerfiles, along with their explanations and usage.

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Dockerfile Keywords and Instructions

1. FROM

• Usage: FROM <image>:<tag>
• Description: Defines the base image for the Docker image. Every Dockerfile must begin with a FROM instruction, specifying the base layer for the new image.
• Example: FROM python:3.9

2. WORKDIR

• Usage: WORKDIR <directory-path>
• Description: Sets the working directory for subsequent instructions like COPY, RUN, and CMD. If the directory doesn’t exist, Docker creates it.
• Example: WORKDIR /app

3. COPY

• Usage: COPY <source> <destination>
• Description: Copies files or directories from the host system into the Docker container’s filesystem.
• Example: COPY . /app

4. ADD

• Usage: ADD <source> <destination>
• Description: Similar to COPY, but with extra capabilities like decompressing .tar files directly into the container.
• Example: ADD package.tar.gz /app

5. RUN

• Usage: RUN <command>
• Description: Executes a command in the container during the image build process, often used to install packages or dependencies.
• Example: RUN apt-get update && apt-get install -y nginx

6. CMD

• Usage: CMD ["executable", "param1", "param2"]
• Description: Specifies the default command to run when the container starts. Only the last CMD instruction in the Dockerfile is used.
• Example: CMD ["python", "app.py"]

7. ENTRYPOINT

• Usage: ENTRYPOINT ["executable", "param1", "param2"]
• Description: Configures a container to run as an executable. Often used in combination with CMD to provide default arguments.
• Example: ENTRYPOINT ["nginx", "-g", "daemon off;"]

8. ENV

• Usage: ENV <key> <value>
• Description: Sets environment variables that can be accessed by processes in the container.
• Example: ENV PORT=8080

9. EXPOSE

• Usage: EXPOSE <port>
• Description: Informs Docker that the container will listen on the specified network ports at runtime. It doesn’t publish the port but serves as documentation and metadata.
• Example: EXPOSE 80

10. VOLUME

• Usage: VOLUME ["<path>"]
• Description: Creates a mount point with the specified path that will be stored outside of the container’s filesystem, useful for persistent data.
• Example: VOLUME ["/data"]

11. ARG

• Usage: ARG <name>[=<default value>]
• Description: Defines a build-time variable that users can pass when building the image. Unlike ENV, ARG is only available during the build.
• Example: ARG VERSION=latest

12. LABEL

• Usage: LABEL <key>=<value>
• Description: Adds metadata to the image in key-value pairs, useful for versioning, licensing, and maintaining other metadata.
• Example: LABEL maintainer="hatim@example.com"

13. USER

• Usage: USER <username or UID>
• Description: Specifies the user to use when running the container. This helps run containers with non-root users for added security.
• Example: USER node

14. HEALTHCHECK

• Usage: HEALTHCHECK [OPTIONS] CMD <command>
• Description: Monitors and checks the health of a container, running specified commands to ensure it’s functioning properly.
• Example: HEALTHCHECK CMD curl -f http://localhost || exit 1

15. ONBUILD

• Usage: ONBUILD <INSTRUCTION>
• Description: Adds a trigger instruction to the image, which is executed when the image is used as a base image in another Dockerfile.
• Example: ONBUILD RUN echo "Trigger Instruction"

16. STOPSIGNAL

• Usage: STOPSIGNAL <signal>
• Description: Sets the system call signal to stop the container, like SIGTERM or SIGKILL.
• Example: STOPSIGNAL SIGKILL

17. SHELL

• Usage: SHELL ["executable", "parameters"]
• Description: Specifies the default shell for RUN commands on Windows and Linux containers.
• Example: SHELL ["powershell", "-command"]

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Example Dockerfile

Below is an example Dockerfile that uses some of the above instructions to create a simple Node.js application image.

# Set the base image
FROM node:14

# Set environment variable
ENV NODE_ENV=production

# Set working directory
WORKDIR /app

# Copy package.json and install dependencies
COPY package*.json ./
RUN npm install

# Copy application source code
COPY . .

# Expose application port
EXPOSE 3000

# Default command to run application
CMD ["node", "app.js"]