This assumes you have installed Docker on Linux and Docker is running.

A Docker image contains the code used to create a Docker container, such as creating a Nginx web server, or a mySQL server, or a home grown app, and the list goes on. In this way, an image is like a template used to create a container. An image is kind of like a virtual machine, but much more light weight, using significantly less storage a memory (containers are usually megabytes in size).

The docker images command can be used to return the list of images.

docker images

Let's say the bar image is untagged.

REPOSITORY  TAG     IMAGE ID      CREATED       SIZE
foo         latest  105b54dc64f1  32 hours ago  196.7 MB
bar         <none>  7bd4bc0f144f  32 hours ago  208.9 MB

The docker tag command can be used to tag an image. In this example, the bar image will be tagged with 1.0.

docker tag bar bar:1.0

The docker images command should now show that bar is tagged with 1.0.

REPOSITORY  TAG     IMAGE ID      CREATED       SIZE
foo         latest  105b54dc64f1  32 hours ago  196.7 MB
bar         1.0     7bd4bc0f144f  32 hours ago  208.9 MB

The docker tag command can also be used to create another image with a different tag name. In this example, another foo image will be created with tag "prior".

sudo docker tag foo:latest foo:prior

The docker images command should now show two foo images, latest and prior.

REPOSITORY  TAG     IMAGE ID      CREATED       SIZE
foo         latest  105b54dc64f1  32 hours ago  196.7 MB
foo         prior   105b54dc64f1  32 hours ago  196.7 MB
bar         1.0     7bd4bc0f144f  32 hours ago  208.9 MB