Introduction: Learning Objectives
After this section, you will be able to
- Identify and describe Docker objects,
- Identify essential Dockerfile commands,
- Explain container image naming format, and
- Describe how Docker uses networks, storage, and plugins.
Docker contains objects such as the
- Storage volumes,
- And other objects, such as plugins and add-ons.
A Dockerfile is a text file that contains instructions needed to create an image. You can create a Dockerfile using any editor from the console or terminal.
Let’s go over several of the essential instructions that Docker provides.
- A Dockerfile must always begin with a FROM instruction that defines a base image.
- Often the base image is from a public repository, like an operating system or a specific language like Go or Node.js.
- The RUN instruction executes commands.
- The CMD instruction defines a default command for execution.
- A Dockerfile should have only one CMD instruction.
- If the Dockerfile has several CMD instructions, only the last CMD instruction will take effect.
A Docker Image is a read-only template with instructions for creating a Docker container.
The Dockerfile provides instructions to build the image.
- Each Docker instruction creates a new layer in the image.
- When you change the Dockerfile and rebuild the image, the Docker engine only rebuilds the changed layers.
- Images can share these layers, which saves a lot of disk space as well as network bandwidth when sending and receiving images.
- When you instantiate this image, you get a running container.
- At this point, a writeable container layer is placed on top of the read-only layers.
- The writeable layer is needed because containers are not immutable as images.
How images are named?
An image name has a unique format that consists of three parts:
- The hostname,
- The repository,
- And the tag.
Hostname: The hostname identifies the image registry.
Repository: A repository is a group of related container images.
Tag: The tag provides information about a specific version or variant of an image.
Consider the image name docker.io/ubuntu:18.04.
The hostname docker.io refers to the Docker Hub registry. When using the Docker CLI, you can exclude the docker.io hostname
The repository name ubuntu indicates an Ubuntu image.
Finally, the tag, shown here as 18.04, represents the installed Ubuntu version.
What is a Docker container?
A Docker container is a runnable instance of an image. You can use the Docker API or CLI to create, start, stop, or delete an image. You can connect to multiple networks, attach storage to the container, or create a new image based on its current state.
Docker keeps containers well isolated from each other and their host machine. When using Docker, networks help isolate container communications. By default, data doesn’t persist when the container no longer exists.
Docker uses volumes and bind mounts to persist data even after a container stops. Plugins, such as storage plugins, provide the ability to connect to external storage platforms.
In this section, you learned that:
- Docker contains objects such as the Dockerfiles, images, containers, networks, storage volumes, and other objects, such as plugins and add-ons.
- Essential Docker instructions include FROM, RUN, and CMD.
- A docker container is a runnable instance of an image.
- An image name format consists of three identifiers: the hostname, the repository, and the tag.
- Docker uses networks to isolate container communications.
- Docker uses volumes and binds mounts to persist data even after a container stops running.
- And, Plugins, such as storage plugins, provide the ability to connect to external storage platforms.