Overview of Docker

Get started with Docker and its basic terminologies

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Docker is an open platform for developing, shipping, and running applications.

• Separate your applications from your infrastructure
• Manage infrastructure in the same ways you manage your applications
• Reduce the delay between writing code and running it in production

Docker Architecture

Image: https://docs.docker.com/get-started/docker-overview/

Docker daemon (dockerd)

• Listens for Docker API requests.
• Manages Docker objects such as images, containers, networks, and volumes.
• Communicate with other daemons to manage Docker services.

Docker client (docker)

• Command Line Interface (CLI) for user to talk to Docker daemon.
• Communicate with more than one daemon.

Docker registries

• Stores Docker images.
• Docker Hub is a public registry that anyone can use.
• Docker uses Docker Hub to look for images by default else use custom registry.
• You can run your own private registry. Used by companies for security reasons.
• docker pull pull the required image from registry and docker push push the image to the registry.

Docker image

• A read-only template with instructions for creating a Docker container like Class in programming.
• Often an image is based on another image, with some additional customization. For example, you can create an Apache web server image based on Ubuntu image.
• You can create your own images or use images created by others.
• To build your own image, you create a Dockerfile with a instructions for defining the steps needed to create the image and run it.
• Images define both what you want to package in application and its dependencies.

Docker container

• A runnable instance of an image like Object in programming.
• By default, container is isolated from other containers and its host machine. The isolation can be controlled.
• You can create, start, stop, move, or delete a container using the Docker API or CLI.
• You can attach storage to it.
• When a container is removed, any changes to its state that are not stored in persistent storage disappear.

Exercise: docker run

Try running following command in your terminal and see what happens.

$ docker run -i -t ubuntu /bin/bash

Try to play around with some of shell by running Linux commands

$ ls
$ ls /home/
$ cd /home/
$ mkdir docker-workshop
$ cd docker-workshop
$ cat > first.txt
The first file in the docker-workshop directory.
[ Press Ctrl+ D ]
$ ls
first.txt
$ cat first.txt
The first file in the docker-workshop directory.

NOTE: You can play around with other linux commands as well.

Instructor

Explain the flags -i, -t and pulling of image. Show exiting from container and running it again.

Dockerfile

• A text document that contains instructions for building Docker image.
• Instructions are usually commands a user could call on the command line to assemble an image.
• Each instruction in the Dockerfile adds a new “layer” to the image.
• Using docker build users can create an automated build that executes several command-line instructions in succession.

Dockerfile Example

FROM node:16-alpine3.11

# create app directory in container
RUN mkdir -p /home/app

# copy local app to container app directory
COPY ./app /home/app

# set default app direcotry in container
WORKDIR /home/app

# install app dependencies
RUN npm install

# run the server
CMD ["node", "server.js"]