custom the http path of the UI

__Use case:__
I want to install several instances of influxdb, with different URLs

__Proposal:__
add an environment variable, that we can pass it in the dockerfile
Example: base_path=/user/test/

Start with the contributing section of README.md, then trace how the UI HTTP path is configured and how Dockerfile environment variables are passed. Done means supporting a configurable base path so multiple InfluxDB instances can use different UI URLs.

Written by the indexing model from the issue text.