Docker Basics

In this chapter, we introduce one of the building blocks of the DevOps toolkit – containers. We are going to explain the differences between virtualization and containers, and then present the advantages and disadvantages of both solutions. Additionally, we are going to present a way to choose between both solutions for a given workload.

The main topics covered in this chapter are as follows:

• Virtualization versus containerization
• Anatomy of Docker
• Docker commands
• Dockerfile
• Docker image registries
• Docker networking

Technical requirements

For this chapter, you will need a Linux system with an installed Docker Engine. We are not going to cover the installation steps here. Different Linux distributions provide Docker in different ways. We are going to use Docker Engine 20.10.23 here. Since in this chapter all examples are very basic, older versions of Docker will most probably work. Still, if you run into issues with following our examples, updating Docker to our version should be your first step in troubleshooting.

Virtualization versus containerization

In this section, we are going to explain what virtualization and containerization are and what the major differences between them are.

Virtualization

Virtualization is a technique of running a complete simulated computer within another computer. Complete means that it mirrors everything a physical computer would have: motherboard, BIOS, processor, hard drives, USB ports, and so on. Simulated means that it is entirely a product of software. This computer does not exist physically, thus it is called virtual. To exist, the virtual machine (VM), as simulated computers are often called, needs a real, physical one to emulate it. The physical machine is called a host or hypervisor.

So, I have a physical computer. It is powerful. Why would I want to run a VM in it? For obvious reasons, the VM will be less powerful than the host: after all, the host requires RAM, CPU, and hard drive space for itself. There is also some small drop in performance...

Anatomy of Docker

Docker comprises several components:

• Command-line utility – Docker
• Host
• Objects
• Registries

The Docker CLI tool – docker – is the main means of managing containers and images. It is used to build images, pull them from the registry, upload them to the registry, run containers, interact with them, set runtime options, and, finally, destroy them. It is a command-line tool that communicates with Docker hosts using an API. By default, it is assumed that the docker command is being invoked on the host, but it is not strictly necessary. One docker CLI tool can manage more than one host.

The host is more interesting. The host runs dockerd – a daemon responsible for actually performing the actions ordered via the docker tool. It is here that container images are stored. The host also provides resources such as networking, storage, and the containers themselves.

The dockerd daemon is the beating heart of the containers...

Docker commands

The Docker command-line interface is a tool that allows users to interact with containers. It provides a set of commands that you can use to build Docker images and create and manage containers, images, networks, and volumes. It interacts with the containerd daemon using a socket file or network.

The most common commands you can use are the following:

• build: This allows you to build a new Docker image using a Dockerfile
• run: This starts a new container
• start: This restarts one or more stopped containers
• stop: This will stop one or more running containers
• login: This is used to gain access to private registries
• pull: This downloads an image or a repository from a registry
• push: This uploads an image or a repository to a registry
• build: This helps create an image from a provided Dockerfile
• images: This lists all images on your machine
• ps: This lists all running containers
• exec: This executes a command in a running container...

Dockerfile

A Dockerfile is essentially a text file with a predetermined structure that contains a set of instructions for building a Docker image. The instructions in the Dockerfile specify what base image to start with (for example, Ubuntu 20.04), what software to install, and how to configure the image. The purpose of a Dockerfile is to automate the process of building a Docker image so that the image can be easily reproduced and distributed.

The structure of a Dockerfile is a list of commands (one per line) that Docker (containerd to be exact) uses to build an image. Each command creates a new layer in the image in UnionFS, and the resulting image is the union of all the layers. The fewer layers we manage to create, the smaller the resulting image.

The most frequently used commands in a Dockerfile are the following:

• FROM
• COPY
• ADD
• EXPOSE
• CMD
• ENTRYPOINT
• RUN
• LABEL
• ENV
• ARG
• VOLUME
• USER
• WORKDIR

You can find a complete...

Docker image registries

A Docker image registry hosts Docker images. Docker images are organized by tags that can be accessed and downloaded by users. These images can be used to create and run containers on a host machine. Image repositories can be hosted either locally or on a remote server, such as on Docker Hub, which is a public repository provided by Docker. You can also create your own private image repositories to share and distribute your images within your organization.

When you pull an image from a Docker image repository, the image is composed of multiple layers. Each layer represents an instruction in the Dockerfile that was used to build the image. These layers are stacked on top of each other to create the final image. Each layer is read-only and has a unique ID.

Thanks to UnionFS, the Docker registry shares common layers between multiple images and containers, reducing the amount of disk space required. When a container modifies a file, it creates a new layer...

Docker networking

There are four types of Docker networking: none, bridge, host, and overlay.

Bridge is the default network mode in Docker. Containers in the same bridge network can communicate with each other. Shortly, it creates a virtual network, in which containers are assigned IP addresses and can cummunicate using them, while anything outside of that network cannot reach any of those addresses. In the Host network, the container uses the host’s network stack. This means that the container shares your machine’s IP address and network interfaces.

Overlay mode allows you to create a virtual network that spans multiple Docker hosts. Containers in different hosts can communicate with each other as if they are on the same host. It’s useful when running Docker Swarm.

Using the Docker command line, you are able to create a custom network of any of those types.

None network

A none network in Docker is a special type of network mode that disables all...

Summary

In this chapter, we have introduced one of the major building blocks of modern DevOps-led infrastructure, that is, containers. We described the most prominent container technology – Docker. We have also introduced the basics of running Docker containers and building your own. In the next chapter, we are going to build on this knowledge and introduce more advanced Docker topics.