Docker - A Complete Guide For Beginners

What is Docker?

A service for managing containers is called Docker. Docker is an open platform for developing, shipping, and running applications. The entire purpose of Docker is to make it simple for developers to create apps, load them into containers, and then use them wherever.

Container

• With Docker, you can bundle and execute an application in a loosely isolated container. Containers are lightweight and contain everything needed to run the application.

• Develop your application and its supporting components using containers.

• The container becomes the unit for distributing and testing your application.

• When you're ready, deploy your application into your production environment, as a container or an orchestrated service.

• This works the same whether your production environment is a local data center, a cloud provider, or a hybrid of the two.

What does Docker get used for?

Quick and consistent delivery of your Application

Docker simplifies the development lifecycle By enabling developers to operate in standardized settings with local containers that host your applications and services. Containers are great for continuous integration and continuous delivery (CI/CD) workflows.

Example Scenarios:

• Developers write code locally and share their work with their colleagues using Docker containers.

• They execute both automatic and manual tests on their applications by pushing them into a test environment using Docker.

• When developers find bugs, they can fix them in the development environment and redeploy them to the test environment for testing and validation.

• When testing is complete, getting the fix to the customer is as simple as pushing the updated image to the production environment.

Docker Desktop

Docker Desktop is an easy-to-install application for your Mac, Windows, or Linux environment that enables you to build and share containerized applications and microservices. Docker Desktop includes the Docker daemon (dockerd), the Docker client (docker), Docker Compose, Docker Content Trust, Kubernetes, and Credential Helper.

Docker Images - Template to create a docker container.

An image is a read-only template with instructions for creating a Docker container. Often, an image is based on another image, with some additional customization. For example, you may build an image that is based on the Ubuntu image but installs the Apache web server and your application, as well as the configuration details needed to make your application run.

You might create your images or you might only use those created by others and published in a registry. To build your image, you create a Dockerfile with a simple syntax for defining the steps needed to create the image and run it. Each instruction in a Dockerfile creates a layer in the image. When you change the Dockerfile and rebuild the image, only those layers that have changed are rebuilt. This is part of what makes images so lightweight, small, and fast when compared to other virtualization technologies.

Docker Containers - Running instance of docker image

A container is a runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI. You can connect a container to one or more networks, attach storage to it, or even create a new image based on its current state.

By default, a container is relatively well isolated from other containers and its host machine. You can control how isolated a container's network, storage, or other underlying subsystems are from other containers or the host machine.

A container is defined by its image as well as any configuration options you provide to it when you create or start it. When a container is removed, any changes to its state that aren't stored in persistent storage disappear.

Docker Registries

A Docker registry stores Docker images. Docker Hub is a public registry that anyone can use, and Docker looks for images on Docker Hub by default. You can even run your private registry.

When you use the docker pull or docker run commands, Docker pulls the required images from your configured registry. When you use the docker push command, Docker pushes your image to your configured registry.

Docker objects

When you use Docker, you are creating and using images, containers, networks, volumes, plugins, and other objects. This section is a brief overview of some of those objects.

Docker File

Text documents contain all the commands that a user can call on the command line to assemble an image.

The Process:

Docker file → Build → Docker image → Run → Docker Container

Example docker run command

The following command runs an Ubuntu container, attaches interactively to your local command-line session, and runs /bin/bash.

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

When you run this command, the following happens (assuming you are using the default registry configuration):

• If you don't have the Ubuntu image locally, Docker pulls it from your configured registry, as though you had run Docker pull ubuntu manually.

• Docker creates a new container, as though you had run a docker container create command manually.

• Docker allocates a read-write filesystem to the container, as its final layer. This allows a running container to create or modify files and directories in its local filesystem.

• Docker creates a network interface to connect the container to the default network since you didn't specify any networking options. This includes assigning an IP address to the container. By default, containers can connect to external networks using the host machine's network connection.

• Docker starts the container and executes /bin/bash. Because the container is running interactively and attached to your terminal (due to the -i and -t flags), you can provide input using your keyboard while Docker logs the output to your terminal.

• When you run exit to terminate the /bin/bash command, the container stops but isn't removed. You can start it again or remove it.