More efficiently check on Docker network monitor status

This avoids the round-trip delay and general overhead of pinging Docker
for availability every time, when we know it (more or less) must be
available if the network monitor is still up and running.

Author: pimterry

src/interceptors/docker/docker-networking.ts

@@ -80,6 +80,7 @@ const dockerNetworkMonitors: { [proxyPort: string]: DockerNetworkMonitor | undef
  * explicitly shut down with stopMonitoringDockerNetworkAliases for this proxy port.
  */
 export async function monitorDockerNetworkAliases(proxyPort: number): Promise<DockerNetworkMonitor | undefined> {
+    if (dockerNetworkMonitors[proxyPort]) return dockerNetworkMonitors[proxyPort];
     if (!await isDockerAvailable()) return undefined;
 
     if (!dockerNetworkMonitors[proxyPort]) {