What and Why of Docker ?

• Virtualization software
• Makes developing and deploying applications much easier
• Packages application with all the necessary dependencies, configuration, system tools and runtimes
• Portable artifact, easily shared and distributed

Docker vs Virtual Machine

• An OS is made up of 2 layers i.e. OS Kernel and OS Application Layer
• Kernel is at the core of every OS
• Kernel interacts between hardware and software components
• Docker contains the OS application layer
• Services and apps are installed on top of that layer
• VMs contains both OS application layer and OS kernel layer
• Docker images are much smaller (Docker images are in MD while VM images in GB)
• Containers take seconds to start while VMs take minutes to start
• Docker is compatible only with Linux distros while VMs are compatible with all OS
• Docker was originally made for Linux
• However, Docker Desktop allows you to run Linux containers on Windows or MacOS
• Docker Desktop uses a Hypervisor layer with a lightweight Linux distro

Install Docker Locally

• Got to the official Docker website
• Follow installation steps

Docker Desktop includes

• Docker Engine
  • A server with a long-running daemon process "Dockerd"
  • Manages images and containers
• Docker CLI - Client
  • CLI to interact with Docker Server
  • Execute Docker commands to start/stop... containers
• GUI Client
  • To manages your containers and images with a GUI

Images vs Containers

Public and Private Registries

Registry vs Repository

Docker Registry

• A service providing storage
• Collection of repositories

Docker Repository

• Collection of related images with same name but different versions

Docker Registries

• A storage and distribution system for Docker images
• Official images available form applications like Redis, Mongo,etc.
• Official images are maintained by the software authors or in collaboration with Docker community
• Docker hosts one of the biggest Docker Registry, called 'Docker Hub'

Image Versioning

• Docker images are versioned
• Different versions are identified by tags

Main Docker commands

docker images #List all docker images

docker ps #List all running containers

#Pull an image from a registry
docker pull {name}:{tag}

docker pull nginx:1.23
#or
docker pull nginx:latest

• To create a container with random name from given image(either local or in dockerhub) and starts it

docker run {name}:{tag}

docker run nginx:1.23

Run container in background and prints the container ID using -d or --detach

docker run -d nginx:1.23

To view logs from service running inside the container (which are present at the time of execution) use :

docker logs {container}
docker logs 1e342f2

To stop a container

docker stop {container}
docker stop 1e342f2

Run Containers

• docker run command Creates a new container
• Doesn't re-use previous container
• use -a pr --all to List all container (stopped and running)

docker ps -a

• to stop a container :

docker stop {container}

• to give name to a container when creating a new container, use:

docker run --name {name} {name}:{tag}

docker run --name web-app nginx:1.23

• to stare one or more stopped containers, use:

docker start {container}

Port Binding

• Application inside container runs in an isolated Docker network
• This allows us to run the same app running on the same port multiple times
• We need to expose the container port to the host (the machine the container runs on)
• use -p or --publish to publish a container's port to the host

docker run -d -p {HOST_PORT}:{CONTAINER_PORT} {name}:{tag}

docker run -d -p 9000:80 nginx:1.23

Create own Image (Dockerfile)

• Dockerfile is text document that contains commands to assemble an image
• Docker can then build an image by reading those instruction

Structure of Dockerfile

• Dockerfile start from a parent image or base image
• You choose the base image, depending on which tool you need to have available, for example node, tomcat, python,etc.
• Dockerfile must begin with a FROM instruction
• Build this image from the specified image
• RUN command will execute any command in a shell inside the container environment
• COPY command copies files or directories from and adds them to the filesystem of the container at the path

COPY {src} {dest}

• While RUN is executed on container, COPY is execute on host
• WORKDIR sets the working directory for all following commands
• Like changing into a directory using cd
• CMD command is for the instruction that is to be execute when a Docker container starts
• There can only be one CMD command in a dockerfile.

Example

• Let our node.js project file structure be

.
├── src/
│   ├── server.js
│   └── Dockerfile
└── package.json

• Then our Dockerfile will be :

FROM node:19-alpine

COPY package.json /app/
COPY src /app/

WORKDIR /app

RUN npm install

CMD ["node","server.js"]

Building the image

• To build the image use the following command:

docker build -t {name}

docker build -t node-app

• The -t or --tag is used to set a name and optionally a tag in the "name:tag" format