What is a Dockerfile?

A Dockerfile is a plain text file that contains a set of instructions to build a Docker image. Each instruction in a Dockerfile creates a new layer in the image.

Docker reads the Dockerfile from top to bottom and executes each instruction to assemble the final image.

Common Dockerfile Instructions

Example 1: Simple Dockerfile for a Node.js App

# Step 1: Use official Node.js base image
FROM node:18-alpine

# Step 2: Set working directory inside container
WORKDIR /app

# Step 3: Copy package.json first (for layer caching)
COPY package.json .

# Step 4: Install dependencies
RUN npm install

# Step 5: Copy all source files
COPY . .

# Step 6: Expose port
EXPOSE 3000

# Step 7: Start the application
CMD ["node", "index.js"]

Build and run:

docker build -t my-node-app .
docker run -d -p 3000:3000 my-node-app

Example 2: Dockerfile for PHP App

FROM php:8.2-apache

WORKDIR /var/www/html

COPY . .

EXPOSE 80

CMD ["apache2-foreground"]

docker build -t my-php-app .
docker run -d -p 8080:80 my-php-app

Example 3: Dockerfile with ENV and ARG

FROM ubuntu:22.04

# Build-time argument
ARG APP_VERSION=1.0

# Runtime environment variable
ENV APP_ENV=production
ENV APP_NAME=OnlineLearner

RUN echo "Building version $APP_VERSION"

CMD ["bash"]

# Pass build argument
docker build --build-arg APP_VERSION=2.0 -t myapp .

CMD vs ENTRYPOINT

# CMD example
CMD ["nginx", "-g", "daemon off;"]

# ENTRYPOINT example
ENTRYPOINT ["nginx"]
CMD ["-g", "daemon off;"]

Best Practices

• Use small base images like alpine to reduce image size.
• Place COPY package.json and RUN npm install before copying source code — this caches dependencies.
• Use .dockerignore file to exclude unnecessary files.
• One process per container — keep containers focused.

Summary

• A Dockerfile is a script of instructions to build a Docker image.
• Key instructions: FROM, RUN, COPY, WORKDIR, CMD, EXPOSE.
• Use docker build -t name . to build the image from the Dockerfile.