Using Docker

What is Docker?

Docker is an open-source containerization platform that allows you to create, deploy, and run applications in containers. Containers allow you to package an application with all of its dependencies and ship it as a single unit, making it easy to deploy and run the application in any environment.

Prerequisites

• To use Docker, you will need to install Docker on your machine. You can download the Docker installer from the Docker website (https://www.docker.com/).

• You will also need a text editor to write your Dockerfiles and a terminal to run the commands.

Getting Started

To get started with Docker, you will need to create a Dockerfile and a docker-compose.yml file.

• A Dockerfile is a text file that contains the instructions for building a Docker image.

• A docker-compose.yml file is a YAML file that defines the services that make up your application and how they should be built, deployed, and scaled.

Writing a Dockerfile

To write a Dockerfile, you will need to specify the base image that you want to use for your application. You can use an official image from the Docker Hub (https://hub.docker.com/) or you can create your own base image.

Here is an example of a Dockerfile that uses the official Node.js image as the base image:

FROM node:12-alpine
WORKDIR /app
COPY package.json .
COPY package-lock.json .
RUN npm install
COPY . .
CMD ["npm", "start"]

This Dockerfile specifies the following instructions:

• FROM: Specifies the base image that the Docker image will be built on top of. In this case, the base image is the official Node.js image with version 12 and the Alpine Linux distribution.

• WORKDIR: Sets the working directory for the application.

• COPY: Copies the specified files from the host machine to the Docker image.

• RUN: Runs the specified command in the Docker image. In this case, it runs npm install to install the dependencies for the application.

• CMD: Specifies the command that will be run when the Docker container is started. In this case, it runs npm start to start the application.

Building a Docker Image

To build a Docker image from a Dockerfile, you will need to use the docker build command.

Here is the syntax for the docker build command:

docker build [OPTIONS] PATH

For example, to build a Docker image from the Dockerfile in the current directory, you can use the following command:

docker build .

This command will build a Docker image using the instructions in the Dockerfile and give it a unique ID.

Running a Docker Container

To run a Docker container from a Docker image, you will need to use the docker run command.

Here is the syntax for the docker run command:

docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

For example, to run a Docker container from the Docker image that you built in the previous step,

you can use the following command:

docker run IMAGE

Replace IMAGE with the ID or name of the Docker image that you want to run.

By default, the docker run command will create a new container from the image and start the application. You can also use the -d flag to run the container in detached mode, which means that the container will run in the background and the command prompt will return immediately.

For example:

docker run -d IMAGE

Viewing Running Containers

To view the containers that are currently running on your machine, you can use the docker ps command.

Here is the syntax for the docker ps command:

docker ps [OPTIONS]

For example, to view all running containers, you can use the following command:

docker ps

To view all containers (running and stopped), you can use the -a flag:

docker ps -a

Stopping a Container

To stop a running Docker container, you can use the docker stop command.

Here is the syntax for the docker stop command:

docker stop CONTAINER

Replace CONTAINER with the ID or name of the container that you want to stop.

For example:

docker stop CONTAINER

Removing a Container

To remove a Docker container, you can use the docker rm command.

Here is the syntax for the docker rm command:

docker rm CONTAINER

Replace CONTAINER with the ID or name of the container that you want to remove.

For example:

docker rm CONTAINER

Using docker-compose

In addition to using the docker commands to build and run containers, you can also use docker-compose to simplify the process.

docker-compose is a tool that allows you to define the services that make up your application in a docker-compose.yml file and then run them with a single command.

Here is an example docker-compose.yml file for a simple Node.js application:

version: "3"
services:
  app:
    build: .
    command: npm start
    volumes:
      - .:/app
    ports:
      - "3000:3000"

This file defines a single service, app, which is built from the Dockerfile in the current directory and runs the npm start command. It also mounts the current directory as a volume in the container and exposes port 3000.

To build and run the services defined in a docker-compose.yml file, you can use the docker-compose up command.

Here is the syntax for the docker-compose up command:

docker-compose up [OPTIONS] [SERVICE...]

For example, to build and run the app service defined in the docker-compose.yml file above, you can use the following command:

docker-compose up

This command will build the app service and run it in a container.

To stop the services and remove the containers, you can use the docker-compose down command.

Here is the syntax for the docker-compose down command:

docker-compose down [OPTIONS] [SERVICE...]

For example:

docker-compose down

This command will stop the app service and remove the container.

Additional Resources

For more information on using Docker, you can refer to the Docker documentation (https://docs.docker.com/).ddocker