🐳 Dockerfile Instructions Overview – A Beginner’s Guide to Building Docker Images

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🧲 Introduction – Why Learn Dockerfile Instructions?

When working with containers, Dockerfiles are your blueprint for creating consistent, reproducible environments. Whether you’re packaging a web app, microservice, or API, everything begins with a Dockerfile that defines how your image is built and runs.

Each instruction in a Dockerfile plays a specific role—from selecting the base image to running the final command. Mastering these instructions allows you to streamline development and automate deployments across platforms.

🎯 In this article, you’ll learn:

• 📜 What a Dockerfile is
• 🔧 Overview of all major Dockerfile instructions
• 🧠 When and why to use each command
• ✅ Best practices to write efficient Dockerfiles
• 🙋 Answers to frequently asked questions

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📜 What is a Dockerfile?

A Dockerfile is a plain text file that contains a series of instructions used by Docker to automate image creation. Instead of manually configuring your container every time, you write a Dockerfile once—and Docker builds the environment automatically.

Think of it like a recipe:

• Dockerfile = the recipe
• Docker image = the cooked dish
• Docker container = the dish served on the table

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🔧 Common Dockerfile Instructions Explained

Here’s a quick overview of frequently used Dockerfile commands and what they do:

🧱 Instruction	🔍 Purpose
FROM	Specifies the base image your image is built on (e.g., FROM ubuntu:20.04)
LABEL	Adds metadata to the image (e.g., LABEL maintainer="you@example.com")
ENV	Sets environment variables in the image (e.g., ENV PATH=/app/bin:$PATH)
RUN	Executes commands in the shell during build time (e.g., installing packages)
COPY	Copies files from your local system into the Docker image (COPY . /app)
ADD	Like COPY but also supports remote URLs and extracts archives
CMD	Specifies the default command to run when the container starts
ENTRYPOINT	Sets the main command to run in the container, often combined with CMD
WORKDIR	Sets the working directory for subsequent instructions
EXPOSE	Tells Docker which port the container listens on at runtime (EXPOSE 80)
VOLUME	Creates a mount point for persistent storage inside the container
USER	Specifies which user to use when running the container (USER node)
ARG	Defines build-time variables (values used during docker build)
ONBUILD	Adds a trigger instruction that runs when the image is used as a base for another build

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🧠 Practical Use Cases of Key Instructions

Let’s understand when to use some of the most important commands:

FROM

Defines the starting point of your image. Always the first instruction in your Dockerfile.

Example:

FROM node:18

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RUN

Used to install packages or perform configuration during the image build.

Example:

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

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COPY vs ADD

• COPY is preferred for basic file copying.
• ADD is helpful when unpacking .tar archives or fetching remote files.

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CMD vs ENTRYPOINT

• CMD provides default arguments that can be overridden.
• ENTRYPOINT sets the command that always runs when the container starts.

Example:

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

➡️ This runs python app.py when the container starts.

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ARG vs ENV

• Use ARG for values needed at build time (e.g., versions).
• Use ENV for values required at runtime (e.g., API keys, paths).

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✅ Best Practices for Writing Dockerfiles

✔️ Practice	💡 Why It Matters
Start from minimal base images	Use alpine, slim, etc. to reduce image size
Combine RUN instructions	Minimizes image layers and speeds up builds
Use .dockerignore	Prevents unnecessary files from being copied into the image
Avoid storing secrets	Use Docker secrets or environment variables instead
Label your images	Add metadata to track versions and maintainers
Use explicit tags	Always use specific versions instead of latest for reproducibility

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📌 Summary – Recap & Next Steps

Understanding Dockerfile instructions is essential for creating efficient, secure, and scalable Docker images. Each command in a Dockerfile has a unique role, and mastering them gives you full control over your application’s environment.

🔍 Key Takeaways:

• A Dockerfile is a script that defines how to build a Docker image
• Use FROM, RUN, COPY, CMD, and ENTRYPOINT wisely
• Know the difference between ENV vs ARG and COPY vs ADD
• Optimize your Dockerfiles using best practices

⚙️ Next Steps:
Experiment with writing your own Dockerfiles, explore multi-stage builds, and start containerizing real-world applications.

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🙋 Frequently Asked Questions (FAQs)

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❓ What is the difference between CMD and ENTRYPOINT?
✅ CMD provides default commands that can be overridden when running a container.
✅ ENTRYPOINT defines a fixed executable that always runs.
💬 You can combine them to allow dynamic arguments while locking the executable.

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❓ When should I use COPY vs. ADD?
✅ Use COPY for local files.
✅ Use ADD if you need to extract archives or fetch from URLs.

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❓ Can I use multiple RUN instructions?
✅ Yes, but combine them using && to reduce image layers and improve performance.
Example:

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

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❓ What’s the difference between ARG and ENV?
✅ ARG is available only during build time and is not stored in the final image.
✅ ENV persists inside the container at runtime and is accessible during execution.

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❓ How can I make my Dockerfile more efficient?

• Use minimal base images like alpine
• Combine RUN commands
• Use .dockerignore
• Avoid unnecessary packages or build tools

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