When creating a container using the docker run command, the -h or --hostname option can be used to define the hostname of the container. 

In this example, a container is created using the foo:latest image, and the hostname of the container will be foo.example.com.

docker run --hostname foo.example.com foo:latest

The docker container ls command can be used to ensure the container is up and running.

~]# docker container ls
CONTAINER ID     IMAGE              COMMAND                 CREATED         STATUS     PORTS     NAMES
d937372c09ab9    b939aa938add9913   "/docker-entrypoin..."  6 minutes ago   Up 6 minutes         foo

The docker exec command can be used to ensure the container has the hostname you declared.

~]# docker exec foo hostname
foo.example.com