Move all .env variables into docker-compose.yml

[MicahZoltu]

While Docker does support config files, it is often undesireable to have configuration in two places (docker-compose and .env). For the docker-compose instructions, all of the necessary environment variables and their comments should be migrated to the docker-compose file, and the instructions updated to explain how to set them. It may be worth making a note which environment variables can be read from a .env file for those who prefer that method for configuration.

Importantly, if all configuration is read from the compose file, then you can publish a pre-built image to Docker Hub or GitHub image repository and users would no longer need to clone the repository to get OpenCast working in Docker. They would merely need to copy/paste the docker-compose.yml file into their management software or a file on their desktop. The inclusion of the .env creates a requirement to build from source, which is unfortunate for many users.