exec: rework API to get exit code

src/nginxconfig.ejs

@@ -216,7 +216,7 @@ server {
     }
 
     # the read timeout is between successive reads and not the whole connection
-    location ~ ^/api/v1/apps/.*/exec$ {
+    location ~ ^/api/v1/apps/.*/exec/.*/start$ {
         proxy_pass   http://127.0.0.1:3000;
         proxy_read_timeout 30m;
     }