# Docker Syntax

## **Dockerfile** – *Build Blueprint*

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

**Basic Syntax**:

```bash
# 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.

***

## **docker-compose.yml** – *Multi-Container Config*

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

**Basic Syntax**:

```bash
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.