Mechanics of Running Docker.

The command

docker images

returns a list of available (local) images.

Running a docker image is like running the image as another image (which is called a container to avoid the confusion).

To run an image

Tag of a docker image

Every image has a tag which by default is latest. An image with a number of tags can be thought of as different variants of the image.

An id of an image corresponds to a container of the image.

Thus,

containers are created from images
when you run an image, i.e., inside a container, and make changes like create files or install softwares, the changes will disappear if container after exit is not commited.

Note:

docker ps -a

docker ps -l

docker commit <container_name> ubuntu:latest updates the source image by writing over it.
docker commit <container_name> ubuntu:with_my_file updates the source image by creating a new variant defined by its tag.
docker commit <container_name> changed_ubuntu creates a new image based on the source image which has the tag latest by default.