Set custom host via docker param or query string

[jonfinerty]

Hi, thanks for adding the ability to change host in the UI in #19. I have a very similar use case to #13 and I'd love to point the pubsub UI directly to the desired port on start up rather than needing to remind other developers in my org to update the setting in the UI as that's easy to forget to do.

What would be fantastic is if I could either set this when running the docker container as a parameter, or in a query string like &host=localhost:8099. So I can automate this configuration away for my end users.

[NeoScript]

Hmm this query string idea might be very good. From my understanding the env var option listed in #26 does not really work since angular cant pull environment variables during run time without some significant changes. I was playing around with building a cli tool for pubsub emulator that this UI would tap into, but let me take a shot at just doing it via query params. I'll try it and get back to ya if it works 👍