When several applications share a machine, their dependencies and resource needs can interfere with one another. A container runs an application and its dependencies in an isolated environment without requiring a separate operating system kernel for each application. This makes containers lighter than virtual machines while retaining much of their isolation. section 1.1 section 1.1.1
An image and a container are not the same thing. An image is a file artifact, either downloaded or built locally. A container instance—usually called a container—is one invocation of that image. Each local Docker invocation creates an instance that remains Docker-managed after its process stops, until it is cleaned up. section 2.1.2
Try an isolated shell
Run `docker run -it ubuntu bash` to start an interactive shell from the Ubuntu image. Commands in that shell run inside the container: installing Python there does not install it on your local system. Exit the shell to stop the container; the stopped instance remains available. section 2.1.2
$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS c5e023cab033 ubuntu "bash" 5 minutes ago Exited (0) 1 second ago $ CONTAINER_ID=c5e023cab033 $ docker start $CONTAINER_ID $ docker attach $CONTAINER_ID # echo "run more commands" # exit
The command sequence lists a stopped container, starts it by ID, and attaches to its shell again. It illustrates why stopping a local container is different from removing it. section 2.1.2
Containers also separate projects with different languages, libraries, or package versions. A Dockerfile records how to build the image, helping reproduce the environment in development and production. Local container instances still need housekeeping: repeated runs can leave many stopped containers using disk space. section 1.1.1 section 2.0 section 2.1.2
In Kubernetes, containers run inside Pods. A Pod may contain one container or several scheduled together; a Deployment can request multiple replicas of a Pod. Kubernetes configuration names images rather than persistent Docker instances: the container instances are created at runtime and deleted when their Pod stops. section 1.2.1 section 2.1.2
