Docker Syntax

Dockerfile – Build Blueprint

A Dockerfile contains step-by-step instructions to create a Docker image.

Basic Syntax:

# Base image
FROM node:18

# Set working directory
WORKDIR /app

# Copy files
COPY package*.json ./
RUN npm install
COPY . .

# Expose port and run app
EXPOSE 3000
CMD ["node", "index.js"]

Common Keywords:

• FROM – base image (e.g., ubuntu, node, python).

• COPY – copies files from host to container.

• RUN – executes a command during build (e.g., install dependencies).

• CMD – defines the default command to run when container starts.

• WORKDIR – sets the working directory inside the container.

• EXPOSE – tells Docker what port the container listens to.

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docker-compose.yml – Multi-Container Config

docker-compose.yml is used to define and run multi-container applications.

Basic Syntax:

version: '3.8'

services:
  web:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/app
    depends_on:
      - db

  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb

Common Parts:

• version: Compose file format version.

• services: Defines the containers (e.g., web, db).

• build: Builds from a Dockerfile.

• image: Uses an image from Docker Hub.

• ports: Maps container to host ports.

• volumes: Mounts host paths or named volumes.

• environment: Set environment variables.

• depends_on: Set container startup order.