In Dockerland, there are Registries, images and there are containers. The three are closely intertwined, but distinct.
An image is worth a thousand words
What is this Docker registry?
The Registry is a stateless, highly scalable server side application that stores and lets you distribute Docker images. - https://docs.docker.com/registry/
- Docker offers the community a public official registry: https://hub.docker.com
- An organization can install it's private registry such as ours.
What is a Docker image?
An image is an inert, immutable, file that's essentially a snapshot of a container. Images are created with the build command, and they'll produce a container when started with
On the command line, one could do the same:
docker search ubuntu
NAME DESCRIPTION STARS OFFICIAL AUTOMATED ubuntu Ubuntu is a Debian-based Linux operating s... 4846 [OK] ubuntu-upstart Upstart is an event-based replacement for ... 66 [OK] rastasheep/ubuntu-sshd Dockerized SSH service, built on top of of... 42 [OK] ubuntu-debootstrap debootstrap --variant=minbase --components... 28 [OK]
Let's run the classical hello world for Docker:
docker run hello-world
If you can see the below, this means that it works. As you can see, behind the scene, Docker did a lot to make this work:
Hello from Docker! This message shows that your installation appears to be working correctly. To generate this message, Docker took the following steps: 1. The Docker client contacted the Docker daemon. 2. The Docker daemon pulled the "hello-world" image from the Docker Hub. 3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading. 4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal. To try something more ambitious, you can run an Ubuntu container with: $ docker run -it ubuntu bash Share images, automate workflows, and more with a free Docker Hub account: https://hub.docker.com
(We can also run the ubuntu version of hello world by simply doing:
docker run ubuntu /bin/echo hello world)
What is this Docker container?
To use a programming metaphor, if an image is a class, then a container is an instance of a class—a runtime object. Containers are hopefully why you're using Docker; they're lightweight and portable encapsulations of an environment in which to run applications.
Container and Virtualization side by side
To recap, Here we saw words like, docker hub/registry, images, containers...
- a registry can be public or private, and usually publish images
- images can be pulled, pushed to a registry. images are build from Dockerfile's
- containers can be created from images, by running a given image