🐳 Dockerfile Instructions Overview – A Beginner-Friendly Guide with FAQs

A Dockerfile is a text file that contains a list of instructions for building a Docker image. Think of it as a blueprint for creating a containerized environment. Each line in a Dockerfile tells Docker how to assemble the image, what to install, and how to run it.

In this article, we’ll go over the most commonly used Dockerfile instructions, offering a brief overview of each to help you understand what they do.

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📜 What is a Dockerfile?

A Dockerfile automates the process of building Docker images. Instead of manually installing software and setting up configurations inside a container, you describe everything in a Dockerfile, and Docker builds the image from it.

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🔧 Common Dockerfile Instructions

Here’s a breakdown of frequently used Dockerfile instructions:

🧱 Instruction	🔍 Purpose
FROM	Sets the base image for the Docker image (e.g., FROM ubuntu:20.04)
LABEL	Adds metadata to the image (e.g., LABEL maintainer="you@example.com")
ENV	Sets environment variables (e.g., ENV PATH=/app/bin:$PATH)
RUN	Executes commands in the shell inside the image (e.g., RUN apt-get update)
COPY	Copies files from the host machine into the Docker image (e.g., COPY . /app)
ADD	Similar to COPY, but also handles remote URLs and automatic unpacking of archives
CMD	Sets the default command to run when a container starts (e.g., CMD ["node", "app.js"])
ENTRYPOINT	Defines a command that always runs in the container; can be combined with CMD
WORKDIR	Sets the working directory for RUN, CMD, ENTRYPOINT (e.g., WORKDIR /app)
EXPOSE	Informs Docker that the container will listen on a specific port (e.g., EXPOSE 80)
VOLUME	Creates a mount point for persistent storage (e.g., VOLUME /data)
USER	Specifies which user to run the container as (e.g., USER node)
ARG	Defines build-time variables (e.g., ARG VERSION=1.0)
ONBUILD	Sets a trigger instruction to run when the image is used as a base for another build

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✅ Conclusion

Dockerfile instructions are the heart of Docker image creation. Once you understand the role of each instruction, you can easily craft your own Dockerfiles to suit your application needs. Start small, experiment, and build up as you gain confidence!

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🙋 Frequently Asked Questions (FAQs)

❓ What is the difference between CMD and ENTRYPOINT?

• CMD provides defaults that can be overridden when running a container.
• ENTRYPOINT sets a command that always runs and is not easily overridden.
• You can use both together to provide flexible and controlled command execution.

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❓ When should I use COPY vs. ADD?

• Use COPY for simple file copying.
• Use ADD only when you need features like extracting archives or downloading from URLs.

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❓ Can I have multiple RUN instructions?

Yes, but it’s a best practice to combine them into fewer layers using && to minimize image size.

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

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❓ What is the role of ARG and how is it different from ENV?

• ARG is used at build time, meaning its value is not preserved in the final image.
• ENV is used at runtime, and its values are preserved inside the container.

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❓ How can I make my Dockerfile more efficient?

• Use a smaller base image (e.g., alpine).
• Combine RUN statements.
• Use .dockerignore to skip unnecessary files.
• Minimize image layers.

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