Fix pid permission denied errors

See: https://github.com/nginx/docker-nginx-unprivileged/blob/main/README.md#troubleshooting-tips

Author: zacblazic

config/main.conf

@@ -1,5 +1,7 @@
 daemon off;
 
+pid /tmp/nginx.pid;
+
 worker_processes        auto;
 worker_rlimit_nofile    8192;
 worker_shutdown_timeout 240s;