ENV, unlike some of these other commands, should be mostly self-explanatory: it sets the environmental variables both in the Dockerfile and the container. Since we would need to keep re-typing /srv/www/html in our Dockerfile, in order to prevent typos and to ensure easy changes to our final server directory target, we set the SRV_PATH variable that we keep reusing with $SRV_PATH later. Generally for Docker containers, almost all the configurations to containers are done through environmental variables such as these, so expect to see this directive more in the later chapters.
Even though we don't use it in this example, you need to watch out when using environment variables in the CMD directive directly as it does not get expanded but runs directly. You can ensure that your variable gets expanded in CMD by using it as part of a shell command structure similar to this: CMD [ "sh", "-c", "echo", "$SRV_PATH" ].