Add check for nginx pid file in test docker images (#1166)

* add check for nginx pid file

* add alpine default path /var/run/nginx/nginx.pid

Author: sean-breen

test/docker/entrypoint.sh

@@ -33,12 +33,12 @@ echo "starting nginx ..."
 nginx_pid=$!
 
 SECONDS=0
-
-while ! ps -ef | grep "nginx: master process" | grep -v grep; do 
+while [[ ! -f /var/run/nginx.pid ]] && [[ ! -f /var/run/nginx/nginx.pid ]]; do
     if (( SECONDS > 30 )); then
         echo "couldn't find nginx master process"
         exit 1
     fi
+    sleep 1
 done
 
 cat /etc/nginx-agent/nginx-agent.conf;