Dockerfile Instructions Guide

This guide provides a comprehensive overview of the commonly used Dockerfile instructions with examples to help you create efficient and optimized Docker images.

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1. FROM

Specifies the base image for the Docker image.

FROM ubuntu:latest

• Always the first instruction in a Dockerfile.

• Example: FROM python:3.9 for Python-based projects.

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2. WORKDIR

Sets the working directory inside the container.

WORKDIR /app

• Ensures subsequent commands (like COPY or RUN) operate in this directory.

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3. COPY

Copies files or directories from the build context into the container.

COPY src/ /app/

• Use COPY for local files.

• It's simpler and faster than ADD when no archive extraction or URL downloads are required.

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4. ADD

Copies files or downloads remote resources. Also extracts archives.

ADD myfile.tar.gz /app/

• Automatically extracts compressed files (e.g., .tar.gz).

• Supports downloading files from remote URLs.

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5. RUN

Executes commands during the build process (e.g., installing software).

RUN apt-get update && apt-get install -y curl

• Each RUN instruction creates a new image layer.

• Combine commands with && to minimize layers.

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6. CMD

Specifies the default command to run when the container starts.

CMD ["python", "app.py"]

• Can be overridden at runtime using docker run <image> <command>.

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7. ENTRYPOINT

Defines the main application or command to run in the container.

ENTRYPOINT ["nginx", "-g", "daemon off;"]

• Works with CMD to provide default arguments.

• Use ENTRYPOINT for containers meant to act like executables.

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8. ENV

Sets environment variables.

ENV NODE_ENV=production

• Variables can be accessed inside the container.

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9. EXPOSE

Documents the port(s) the container listens on.

EXPOSE 8080

• Does not publish the port; you need -p in docker run.

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10. LABEL

Adds metadata to the image.

LABEL maintainer="[e-mail]"

• Use for descriptions, versioning, or maintainers.

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11. ARG

Defines build-time variables.

ARG APP_VERSION=1.0

• Passed during the build process (docker build --build-arg APP_VERSION=2.0).

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12. VOLUME

Creates a mount point for persistent data.

VOLUME /data

• Ensures the directory persists outside the container.

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13. USER

Specifies the user for running container processes.

USER appuser

• Use to improve security by avoiding root privileges.

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14. SHELL

Changes the shell used in RUN commands.

SHELL ["/bin/bash", "-c"]

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15. ONBUILD

Sets triggers for dependent images.

ONBUILD RUN apt-get update

• Executes only when the image is used as a base for another Dockerfile.

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Best Practices

• Use Multi-Stage Builds: Reduce image size by separating build and runtime stages.

• Minimize Layers: Combine commands in RUN instructions.

• Leverage .dockerignore: Exclude unnecessary files from the build context.

• Use Tags for Base Images: Avoid latest for more consistent builds.

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Example Dockerfile

Here's a complete example:

# Use Python as the base image
FROM python:3.9

# Set environment variables
ENV APP_HOME=/app

# Set the working directory
WORKDIR $APP_HOME

# Copy application files
COPY . $APP_HOME

# Install dependencies
RUN pip install -r requirements.txt

# Expose the port
EXPOSE 5000

# Run the application
CMD ["python", "app.py"]

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Resources

• Dockerfile Reference

• Docker Best Practices