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

sudo docker run --hostname foo.example.com --name my-container my-image:latest

Of if the container already exists the docker update command can be used.

sudo docker update --hostname foo.example.com my-container

You can use the docker inspect command to view the JSON for the container.

~]$ sudo docker inspect my-container | jq .[].Config.Hostname
"foo.example.com"

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

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