[Bug] starting container with vpn takes a lot of time
**Describe the bug**
Starting a container on a node that has vpn enabled takes about 3 minutes more than starting it on the same node with vpn disabled.

**To Reproduce**
Steps to reproduce the behavior:
With vpn_subnet in the node config, start the node, run a task. Use the portal UI to see start and finish time of the task.

Comment out the vpn_subnet line in the config. Repeat the above.
See that without the vpn configured the task takes about 3 minutes less.


diff --git a/vantage6-node/vantage6/node/docker/vpn_manager.py b/vantage6-node/vantage6/node/docker/vpn_manager.py
index cdadf289..a99f85b2 100644
--- a/vantage6-node/vantage6/node/docker/vpn_manager.py
+++ b/vantage6-node/vantage6/node/docker/vpn_manager.py
@@ -350,29 +350,37 @@ class VPNManager(DockerBaseManager):
             return None  # no port assigned if no VPN is available
 
         # Get IP Address of the algorithm container
+        self.log.debug("Getting IP address of algorithm container")
         algo_helper_container.reload()  # update attributes
         algo_ip = self.get_isolated_netw_ip(algo_helper_container)
 
         # Set ports at which algorithm containers receive traffic
+        self.log.debug("Finding exposed ports of algorithm container")
         ports = self._find_exposed_ports(algo_image_name)
 
         # Find ports on VPN container that are already occupied
         cmd = (
             'sh -c '
-            '"iptables -t nat -L PREROUTING | awk \\'{print $7}\\' | cut -c 5-"'
+            '"iptables -t nat -L PREROUTING -n | '
+            'awk \\'{print $7}\\' | cut -c 5-"'
         )
         occupied_ports = self.vpn_client_container.exec_run(cmd=cmd)
+
         occupied_ports = occupied_ports.output.decode('utf-8')
         occupied_ports = occupied_ports.split('\\n')
         occupied_ports = \\
             [int(port) for port in occupied_ports if port != '']
+        self.log.debug(f"Occupied ports: {occupied_ports}")
 
         # take first available port
         vpn_client_port_options = set(FREE_PORT_RANGE) - set(occupied_ports)
         for port in ports:
-            port['port'] = vpn_client_port_options.pop()
+            port_ = vpn_client_port_options.pop()
+            self.log.debug(f"Assigning port {port_} to algorithm port")
+            port['port'] = port_
 
         vpn_ip = self.get_vpn_ip()
+        self.log.debug(f"VPN IP: {vpn_ip}")
 
         # Set up forwarding VPN traffic to algorithm container
         command = 'sh -c "'
['vantage6-node/vantage6/node/docker/vpn_manager.py']