client/nginx: enable Cross-Origin Resource Sharing for API calls

Fixes #275

client/nginx.conf.docker

@@ -32,9 +32,19 @@ http {
         }
 
         location ~ ^/api/(.*)$ {
+            add_header 'Access-Control-Allow-Origin' '*';
+            if ($request_method = 'OPTIONS') {
+                add_header 'Access-Control-Allow-Methods'
+                    'GET, POST, PUT, DELETE, OPTIONS';
+                add_header 'Access-Control-Allow-Headers'
+                    'Authorization, Content-Type';
+                return 200;
+            }
+
             if ($request_uri ~* "/api/(.*)") {
                 proxy_pass http://backend/$1;
             }
+
             gzip on;
             gzip_comp_level 3;
             gzip_min_length 20;
@@ -50,9 +60,11 @@ http {
         location / {
             root /var/www;
             try_files $uri /index.htm;
+
             gzip_static on;
             gzip_proxied expired no-cache no-store private auth;
         }
     }
 }
+
 daemon off;