docker config via env vars

Allows setting config options via env vars in docker.

For example:
`docker run -p 8080:8080 -e DOCKER_ORG_NAME=Nordaaker -e DOCKER_INVITE_CODE=foo nordaaker/convos`

(If of interest, a next step could be splitting up into separate Docker containers for frontend and backend - ideally a docker container runs just one process instead of multiple via supervisor).