66

Docker Ubuntu Image

A docker image can be described as a file used for executing code in a container of a docker. A docker image acts as a group of instructions for building a docker container such as a template. Also, the docker images act as the beginning point if using docker. In the virtual machine environment, a snapshot is comparable to an image.

Docker is used for creating, running, and deploying applications in the containers. A docker image includes application codes, dependencies, tools, libraries, and other files required to make any application run. It can become many or one instances of a container if a user executes an image.

Docker images contain more than one layer and they originate from the old layer but are distinct from it.

• The layers can speed up the builds of docker while decreasing the use of disk and increasing reusability.
• Also, image layers are read-only files.
• A writable layer is included on the head of the unchangeable images, permitting a user to make modifications once a container is established.
• In docker containers and images, references can be confusing to disk space.
• It is necessary to distinguish between virtual size and normal size.
• Normal size refers to the space of the disk that a writable layer of the container applies, while the virtual size is the space of the disk used for a writable layer and container.
• An image’s read-only layers can be distributed between containers beginning from a similar image.

Use Cases of Docker

A docker image contains everything required to execute a containerized application such as libraries, runtime, environment variables, config files, and code. When an image is deployed to the docker environment, then it could be run as the docker container. The run command of a docker establishes a container from a particular image.

A docker image is a reusable asset and deployable on a host. The developers can get the layers of the static image from a single project and apply them to another. It saves the time of a user because they don’t need to recreate any image using scratch.

Difference between Docker Image Docker Container

A docker container can be described as a virtualized runtime environment that is used inside the development of an application. It is used for creating, running, and deploying applications that are separated through the underlying hardware. The docker containers can use a single machine, distribute its kernel, and virtualize the operating system to execute more separated processes. A docker container is lightweight as a result.

A docker image is the same as a snapshot in other kinds of virtual machine environments. It is a docker container’s record at a particular point in time. Also, the docker images are immutable. They can be deleted, shared, or duplicated, while they cannot be changed. The feature is helpful to test new configurations of software because the image will be unchanged whatever happens.

Containers require a runnable image to remain. They are reliant on images because they are used for constructing runtime environments and are required to execute an application.

Docker Image Anatomy

A docker image contains several layers and all images contain everything required for configuring the environment of a container: dependencies, tools, system libraries, and other files. A few of the image parts are mentioned and explained below:

1. Base image

A user can entirely create this initial layer with the help of the build command from scratch.

2. Parent image

A parent image could be the initial layer within a docker image as a substitute to the base image. it’s a reused image that acts as a foundation for every other.

3. Container layer

The docker images do not just make a new container, but a container or writable layer as well. It hosts modifications made to the active container and saves newly deleted and written files, as well as modifications to the existing files. Also, this layer is used for customizing containers.

4. Layers

Many layers are connected to the base image with the help of the code that will let it execute within a container. All docker image layers are viewable upon the /var/lib/docker/aufs/diff directory or by the history command of the docker in CLI (Command Line Interface).

The default status of the docker is to display every top-layer image such as file sizes, tags, and repository. An intermediate layer is cached, enabling top layers convenient to view. Dockers contain storage drives that manage the image layer content management.

5. Docker manifest

This docker image part is an extra file. It applies JSON format for describing the image with information like digital signature and image tags.

Repositories of Docker Image

A docker image gets stored in public or private repositories like those within the cloud registry service of Docker Hub, from which all users can use containers and share and test images. The Docker Trusted Registry of Docker Hub also offers access control and image management capabilities.

official image is the one that Docker generates, while the community image is an image Docker that users make. CoScale agent can be described as an official image of Docker that checks Docker applications. Docker-dd-agent/Datadog, a container of a Docker for agents inside the log management program of Datadog.

It is an example of the community docker image.

• Also, users can make new images with the help of the existing ones and apply the push command of the Docker for uploading custom images on the Docker Hub.
• Docker facilitates feedback to producers before briefing for ensuring the community images quality.
• The producer is liable for updates once an image is released.
• Producers should be cautious if sourcing any image from other parties because all attackers can get access to a computer from copycat images created for tracking a user into thinking that they are belonging from the trusted sources.
• The latest image concept may cause confusion as well. Docker images titled with “.latest” aren’t essentially the latest in a typical sense.
• The latest tag doesn’t refer to the most currently pushed the release of an image; simply it’s a default tag.

How can we make a Docker Image?

All Docker images could be created from either a Dockerfile or interactive method. These two methods are listed and explained below:

Dockerfile Method

This method needs establishing a plain text Dockerfile. This Dockerfile establishes the specifications to create an image. This procedure is more time-consuming and difficult, but it does very well in the environment of continuous delivery. The method contains establishing the Dockerfile and including the commands required for the image.

The user structures a file, i.e., dockerignore for excluding a file not required for the final frame once the Dockerfile is begun. The file, i.e., Dockerignore resides in the root directory. The build command of Docker is used for creating a Docker image, the name of an image, and the tag is set. The command of Docker images is used for seeing the created image.

Interactive Method

Users execute a container using an existing image of Docker and make any required modifications to the environment manually before saving an image. This method is one of the easiest ways for creating Docker images. The initial step is to open any terminal session and launch Docker.

Then, apply the run command of the Docker, i.e., image_name:tag_name. It begins a shell session along with the container which was launched through the image. Docker applies the most current release of the image if the name of the tag is omitted. The image must occur listed in outcomes after it.

Advantages of interactive Method

It is the simplest and quickest way for creating Docker images. It is idle to test troubleshoot determined dependencies and very date processes.

Drawbacks of interactive Method

It has a complex lifecycle management, needing manual reconfiguration (error-prone) of many live interactive processes. It is easier to design unoptimized images using unnecessary layers.

Commands of Docker Image

There are groups of the primary image commands of Docker according to Docker. These commands are divided into many child commands. Some of these commands are mentioned and explained below:

• Docker image build: It builds the image using a Dockerfile.
• Docker image load: It loads the image using a tar archive or streams to receive or read input (STDIN).
• Docker image inspect: It shows the information on multiple images.
• Docker image prune: It deletes unused images.
• Docker image push: It pushes a repository or image to a registry.
  Docker image pull: It pulls a repository or image through a registry.
• Docker image save: It saves multiple images to the tar archive (by default streamed to STDOUT).
• Docker image rm: It removes multiple images.
• Docker image tag: it makes a TARGET_IMAGE tag that refers to SOURCE_IMAGE.

The command-line interface of Docker offers several commands that are used for customizing many docker images. Some of the examples of the commands of the Docker image are mentioned and explained below:

• Docker images history: It displays an image’s history including modifications apply to it and its layers.
• Docker tag: It makes a tag like the TARGET_IMAGE tag, which enables all users for grouping and organizing container images.
• Docker update: It enables the user for updating the container’s configuration.
• Docker save: It enables the user for saving images to the archive.
• Docker search: It searches inside the Docker Hub for whatever a user requires.
• Docker compose: It is used for handling a variable of an environment.
  A Docker image is an important tool and concept to understand when working in Docker for creating applications within containerized environments.

Image Layers

All files that build up a Docker image is called a layer. These layers build an intermediate image’s series, created one on head of the other within stages, where all layers are dependent on a layer immediately under it. The hierarchy of our layers is the key to convenient lifecycle management of our Docker images.

Parent Image

The initial layer of the Docker images is called the parent image in most of the cases. It is the foundation under which every other layer is created and offers the general building blocks for our container environments. We can see a huge range of ready-made images for application as our parent image over the public container registry Docker Hub.

Also, we can see them over a small number of many third-party services like the Google Container Registry. We can apply one of our existing images as a support to create new ones alternatively. A normal parent image might be a stripped-down distribution of Linux or provides a pre-installed service like a content management system (short for CMS) or a database management system (DBMS).

Base Image

A base image can be described as an empty initial layer in simple terms, which permits us to create our Docker images using scratch. These images provide us complete control over the image’s contents, but are basically designed for more leading users of Docker.

Registries of Container

The registries of container are the storage location’s catalogs, which are called repositories, where we can pull and push container images.

Three of the primary types of registries are listed and discussed below:

• Docker Hub: It is the official image resource of a Docker, in which we can access 100000+ container images distributed by software vendors, user community of Docker, and open-source projects. Also, we can apply the service for hosting and managing our private images.
• Self-hosted registries: It is a registry model recommended by many organizations that mainly prefer for hosting container images on their on-premises infrastructure, generally due to low latency requirements, compliance concerns, or security. We need to use a registry server to run our self-hosted registry. We can also set up our private, virtual, and remote Docker registry alternatively.
• Third-party registry services: This registry is completely managed offerings that provides a central point of approach to our container images, facilitating a way for storing, managing, and securing them without any operational headache of executing on-premises registry. Instances of the third-party registry offerings that provides support for Docker images such as JFrog Container Registry, Google Container Registry, Azure Container Registry, Amazon ECR, and Red Hat Quay.

Dockerfile Method

The Dockerfile method is an approach of choice for enterprise-grade and real-world container deployments. It is a more efficient, flexible, and systematic way for building docker images and a key to secure, reliable, and compact container environments.

The Dockerfile approach in short is a process of three-step whereby we make the Dockerfile and include the commands we need for assembling the image.

The below table displays us those statements of the Dockerfile we are most likely to apply:

Command	Purpose
FROM	This command is used for specifying the parent image.
WORKDIR	It is used for setting the working directory for a command that pursue within the Dockerfile.
RUN	It is used for installing any packages and applications required for our container.
COPY	It is used for copying over directories or files from a particular location.
ADD	It is the same as the COPY command, but also can unpack compressed files and manage remote URLs.
ENTRYPOINT	It is the command that will be always run when the container begins. The default will be /bin/sh/ -c if not specified.
CMD	All arguments will be passed to the entrypoint command. The CMD would be the commands a container executes if the ENTRYPOINT command isn’t set (default is /bin/sh/-c).
EXPOSE	It is used for defining which port from which to access our container application.
LABEL	It is used for adding meta data to any image.

Advantages of Dockerfile Method

It includes repeatable, compact, and clean recipe-based images. It includes convenient lifecycle management and convenient integration into continuous delivery (CD) and continuous integration (CI) processes. Also, this method includes a fair self-documented step’s record taken for assembling the images.

Drawbacks of Dockerfile Method

It is more complicated for beginners. Also, it is very time consuming for creating images using scratch.

---