This guide provides a comprehensive overview of the commonly used Dockerfile instructions with examples to help you create efficient and optimized Docker images.
1. FROM
Specifies the base image for the Docker image.
FROM ubuntu:latest
Always the first instruction in a Dockerfile.
Example:
FROM python:3.9for Python-based projects.
2. WORKDIR
Sets the working directory inside the container.
WORKDIR /app
- Ensures subsequent commands (like
COPYorRUN) operate in this directory.
3. COPY
Copies files or directories from the build context into the container.
COPY src/ /app/
Use
COPYfor local files.It's simpler and faster than
ADDwhen no archive extraction or URL downloads are required.
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.
5. RUN
Executes commands during the build process (e.g., installing software).
RUN apt-get update && apt-get install -y curl
Each
RUNinstruction creates a new image layer.Combine commands with
&&to minimize layers.
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>.
7. ENTRYPOINT
Defines the main application or command to run in the container.
ENTRYPOINT ["nginx", "-g", "daemon off;"]
Works with
CMDto provide default arguments.Use
ENTRYPOINTfor containers meant to act like executables.
8. ENV
Sets environment variables.
ENV NODE_ENV=production
- Variables can be accessed inside the container.
9. EXPOSE
Documents the port(s) the container listens on.
EXPOSE 8080
- Does not publish the port; you need
-pindocker run.
10. LABEL
Adds metadata to the image.
LABEL maintainer="you@example.com"
- Use for descriptions, versioning, or maintainers.
11. ARG
Defines build-time variables.
ARG APP_VERSION=1.0
- Passed during the build process (
docker build --build-arg APP_VERSION=2.0).
12. VOLUME
Creates a mount point for persistent data.
VOLUME /data
- Ensures the directory persists outside the container.
13. USER
Specifies the user for running container processes.
USER appuser
- Use to improve security by avoiding root privileges.
14. SHELL
Changes the shell used in RUN commands.
SHELL ["/bin/bash", "-c"]
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.
Best Practices
Use Multi-Stage Builds: Reduce image size by separating build and runtime stages.
Minimize Layers: Combine commands in
RUNinstructions.Leverage
.dockerignore: Exclude unnecessary files from the build context.Use Tags for Base Images: Avoid
latestfor more consistent builds.
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"]
Resources
Dockerfile Reference
Docker Best Practices
Top comments (1)
bro difference between cmd entrypoint when to use entry when to use cmd