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).

When building an image using the docker build command and a Dockerfile, the USER option can be used to specify the user that the image will run as. 

In this example, user john.doe will issue command echo "Hello World"

FROM centos:latest
USER john.doe
CMD echo "Hello World"