Set basename BrowserRouter prop via environment variable

[jrigassio-splunk]

Hello! I am considering using this project, as it nicely combines swagger apis into one portal, but it would integrate better with our existing traffic routing if I was able to specify a prefix URL to the application (i.e. serve the homepage at /foobar and serve apis at /foobar/petshop and /foobar/instagram).

This is actually quite easy with the basename prop to the BrowserRouter component you are already using. I think it'd be convenient if one could specify this as an environment variable to your Dockerfile. Opening a PR for this that I'd love review on when you're able to take a look.