Set the environment variables you want the Dockerfile to include
What's written in the property gets added to the result dockerfile in a line starting with the ENV instruction. You can either set one variable (VAR=VALUE) or as many as you want to separate them with a space (VAR1=VALUE1 VAR2=VALUE2). Also, make sure you single quote the values that may contain spaces (VAR='THE VALUE').
After the image is created you can check its environment variables executing the following command.
docker inspect <image name> --format {{.ContainerConfig.Env}}
At runtime, you can check every environment variable with the Configuration Manager