What is the usage of variable DRONE_HOST and how to configure it?

Drone Support
1

I want set up drone with reverse proxy by nginx path, such as /drone
How to config this DRONE_HOST variable?

2

Drone does not support subpaths (ie /drone). You need to instead use a subdomain.

There are no immediate plans to implement this capability, however, for updates should something change https://github.com/drone/drone/issues/475