🚀 DevOps Roadmap 2025 – Step 4: Docker (Part 2) – Mastering Dockerfile & Docker Images

In Part 1 of our Docker series, we covered Docker basics, architecture, installation, and important commands.

Now in Part 2, we’ll deep dive into Dockerfile and Docker Images — the building blocks of containerized applications.

📌 1. What is a Dockerfile?

A Dockerfile is a plain text file that contains a set of instructions on how to build a Docker image.
It acts like a recipe — telling Docker exactly what to install, copy, and configure inside the container.

Example:

# This is a sample Dockerfile
FROM ubuntu:20.04
RUN apt-get update && apt-get install -y python3
CMD ["python3", "--version"]

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📌 2. Important Points about Dockerfile Naming

• The correct default name is Dockerfile (with D capital, f small) and no file extension.
  ✅ Correct: Dockerfile
  ❌ Wrong: dockerfile.txt, Dockerfile.txt, dockerFile

• This naming allows Docker to detect it by default when building images.

• If you use another name, you must specify it while building:

    docker build -f MyDockerFile .
  

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📌 3. How to Create a Dockerfile

1️⃣ Create a new directory for your project

mkdir myapp
cd myapp

2️⃣ Create a Dockerfile

touch Dockerfile

3️⃣ Add instructions to your Dockerfile

Instructions must be in uppercase for readability and convention:
Example for a Node.js app:

FROM node:18
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Order matters – Docker builds images layer by layer. Changing a line causes all subsequent layers to rebuild.

• Minimize RUN commands – Combine them to reduce layers.

• Avoid unnecessary files – Use .dockerignore to skip unwanted files like logs, node_modules, or secrets.

📌 4. Dockerfile Instructions Explained

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📌 5. How to Build a Docker Image

docker build -t myapp:1.0 .

• -t → Tags the image (name:version)

• . → Path to Dockerfile (current directory)

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📌 6. How to Run a Docker Image

docker run -it --name mycontainer myapp:1.0

• -it → Interactive terminal

• --name → Container name

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📌 7. Docker Image Management Commands

docker images          # List all images
docker rmi image_id    # Remove image
docker pull ubuntu     # Download image from Docker Hub
docker push myrepo/app # Push image to Docker Hub
docker tag imgid repo/image:tag # Tag image

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📌 8. Best Practices for Writing Dockerfiles

✅ Use official base images
✅ Minimize the number of layers
✅ Use .dockerignore to avoid copying unnecessary files
✅ Combine RUN commands to reduce image size
✅ Use specific versions instead of latest
✅ Keep images small for faster deployments

How to Reduce Docker Image Size

Smaller images mean faster builds, quicker deployments, and less storage usage.

Ways to reduce size:

1. Use smaller base images – Instead of ubuntu, use alpine.

    FROM python:3.9-alpine
   

2. Combine RUN commands –

    RUN apt-get update && apt-get install -y \
        curl \
        git \
     && rm -rf /var/lib/apt/lists/*
   

3. Use .dockerignore – Exclude files you don’t need inside the image.

4. Multi-stage builds – Build in one stage, copy only final output to a lightweight image.

    FROM golang:1.17 AS builder
    WORKDIR /app
    COPY . .
    RUN go build -o myapp
   
    FROM alpine
    COPY --from=builder /app/myapp /myapp
    CMD ["/myapp"]
   

5. Avoid unnecessary packages – Install only what’s needed.

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📌 9. Common Docker Interview Questions (Part 2)

Q1: What is the difference between CMD and ENTRYPOINT?
A: CMD sets default arguments for the container, which can be overridden. ENTRYPOINT defines the fixed executable.

Q2: How does Docker build an image?
A: Docker reads the Dockerfile line by line, creating a new layer for each instruction.

Q3: How can you reduce Docker image size?
A: Use smaller base images, combine RUN commands, use .dockerignore, multi-stage builds, and remove unnecessary packages.

Q4: Difference between COPY and ADD?
A: COPY only copies files/directories. ADD supports remote URLs and automatically extracts archives.

Q5: What is a multi-stage build?
A: A build process where you use multiple FROM statements to separate build and runtime stages, keeping the final image lightweight.

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📌 10. Hands-on Example

Simple Python App
Dockerfile

FROM python:3.10
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

Build & Run

docker build -t python-app .
docker run -it --rm python-app

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💡 Conclusion:
Mastering Dockerfiles and images is essential for creating consistent, portable, and optimized containers.
In the next part, we’ll explore Docker Networking & Volumes.

📢 If you missed Part 1: Docker Basics, read it here → https://harshalsonar.hashnode.dev/devops-roadmap-2025-step-4-mastering-docker-part-1-beginner-to-intermediate-guide