client/build: Clean up build process

Fixes incorrect URIs of iOS splash screens and OpenSans font
Files get gzipped inside build script
Better nginx configuration
build.js uses more consistent, synchronous code

client/nginx.conf.docker

@@ -1,4 +1,5 @@
 worker_processes 1;
+user nginx;
 
 error_log /dev/stderr warn;
 pid /var/run/nginx.pid;
@@ -54,3 +55,4 @@ http {
         }
     }
 }
+daemon off;