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

When creating a container using the docker run command the -e or --env option can be used to define environment variables that will be set in the container. 

sudo docker run --env foo=bar --name my-container my-image:latest

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

sudo docker update --env foo=bar my-container

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

~]$ sudo docker inspect my-container | jq .[].HostConfig.ExtraHosts
foo.example.com:172.16.0.2