Dockerfile vs docker compose

1. Dockerfile

A Dockerfile is a script that contains a set of instructions to create a Docker image. It defines the environment in which an application will run inside a container. The image built from the Dockerfile can then be used to create and run containers..

Dockerfile

# 1. Specify the base image
FROM python:3.9

# 2. Set the working directory
WORKDIR /app

# 3. Copy the dependencies file to the container
COPY requirements.txt .

# 4. Install dependencies
RUN pip install -r requirements.txt

# 5. Copy application source code
COPY . .

# 6. Specify the command to run the application
CMD ["python", "app.py"]

Command	Description
FROM	Specifies the base image (e.g., ubuntu:20.04, node:16)
WORKDIR	Sets the working directory inside the container
COPY	Copies files from the local system to the container
RUN	Executes commands during the image build process (e.g., installing dependencies)
CMD	Specifies the default command to run when the container starts
ENTRYPOINT	Similar to CMD, but allows additional command-line arguments
EXPOSE	Declares the port the container will use
ENV	Defines environment variables

2. docker-compose.yml

A docker-compose.yml file is used to define and manage multiple containers as part of a single application. With docker-compose up, you can start multiple services at once.

Example docker-compose.yml

version: "3.8"

services:
  web:
    build: .
    ports:
      - "5000:5000"
    depends_on:
      - db
    environment:
      - DATABASE_URL=mysql://user:password@db:3306/mydb

  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: mydb
      MYSQL_USER: user
      MYSQL_PASSWORD: password
    ports:
      - "3306:3306"
    volumes:
      - db_data:/var/lib/mysql

volumes:
  db_data:

Key Concepts of docker-compose.yml

Property	Description
version	Specifies the Docker Compose file version (e.g., "3.8")
services	Defines the containers that will be run
build	Uses the Dockerfile to build an image
image	Specifies the Docker image to use
ports	Maps ports between the host and container (host:container)
depends_on	Ensures services start in a specific order
environment	Defines environment variables
volumes	Creates persistent storage

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3. Differences Between Dockerfile and docker-compose.yml

Feature	Dockerfile	docker-compose.yml
Purpose	Defines how to build a container image	Manages multiple containerized services
Usage	docker build, docker run	docker compose up -d, docker compose down
Functionality	Installs dependencies, sets up environment	Defines services, networks, and dependencies

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4. Example Usage

1) Create a Dockerfile

FROM python:3.9
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

2) Create a docker-compose.yml

version: "3.8"

services:
  web:
    build: .
    ports:
      - "5000:5000"
    depends_on:
      - db
    environment:
      - DATABASE_URL=mysql://user:password@db:3306/mydb

  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: mydb
      MYSQL_USER: user
      MYSQL_PASSWORD: password
    ports:
      - "3306:3306"

3) Running the Containers

1. Start the containers using Docker Compose

    docker-compose up -d
   

2. Check running containers

    docker-compose p
   

3. Stop and remove containers

    docker-compose down