The defaults configured for GENERATOR_HOST didn't really make sense.
When running the docker container with `-P`, GENERATOR_HOST defaulted to
http://localhost. This caused download links for generated client/server
code to be incorrect. For most cases, there's no reason to provide
GENERATOR_HOST as the code already figures the appropriate
scheme/host/port from the originating request.

GENERATOR_HOST could still be used for more complex deployment
scenarios, for instance if a specific server is configured as a file
server. I haven't tested this scenario, and it may require mounting /tmp
as a volume when running within a container.