Automatic nginx reloading when config changes

[nazar-pc]

Optional feature should be provided by separate container that will share PID namespace with nginx container and reload nginx process when config gets changed, this simplifies applying config changes, since there is no need to SSH to host and manually restart nginx instance/enter container and ping nginx to gracefully reload config.
Feature should be optional and in separate container.

Currently impossible to implement, needs moby/moby#10163 to be implemented

[nazar-pc]

Can be indeed implemented as background process within the same container (not perfect, but working solution).