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Docker Tutorial — Part 7: Dockerfile & Image Building

Until now, we used prebuilt images. Real engineers build their own images. This is where Docker becomes powerful.

What is a Dockerfile?

A Dockerfile is a blueprint. It tells Docker how to build an image step by step.

Basic Dockerfile Example

FROM node:18

WORKDIR /app

COPY . .

RUN npm install

CMD ["node", "app.js"]

Explanation

• FROM → base image
• WORKDIR → working directory
• COPY → copy files
• RUN → execute commands
• CMD → container start command

Build the Image

docker build -t myapp .

Run the Image

docker run -d myapp

Understanding Image Layers

Each Dockerfile instruction creates a layer. Good Dockerfiles are:

• Small
• Cached
• Secure

Common Beginner Mistakes

• Huge images
• Hard-coding secrets
• Running everything as root

Next, we will learn Docker security and best practices used in production.