Reduce requests required to check that Docker tunnel is running

Previously we checked on the image and then always checked on the
container too. Now, in the very common case where the container is
already running, we just check that and call it a day (we should never
need to pull the image if the container is already running anyway).

Author: pimterry

src/interceptors/docker/docker-tunnel-proxy.ts

@@ -47,16 +47,16 @@ export function ensureDockerTunnelRunning(proxyPort: number) {
     ongoingEnsureTunnelRunningChecks[proxyPort] = containerMutex.runExclusive(async () => {
         const docker = new Docker();
 
-        // Make sure we have the image available (should've been pre-pulled, but just in case)
-        if (!await docker.getImage(DOCKER_TUNNEL_IMAGE).inspect().catch(() => false)) {
-            await pullTunnelImage(docker);
-        }
-
         // Ensure we have a ready-to-use container here:
         const containerName = getDockerTunnelContainerName(proxyPort);
         let container = await docker.getContainer(containerName)
             .inspect().catch(() => undefined);
         if (!container) {
+            // Make sure we have the image available (should've been pre-pulled, but just in case)
+            if (!await docker.getImage(DOCKER_TUNNEL_IMAGE).inspect().catch(() => false)) {
+                await pullTunnelImage(docker);
+            }
+
             await docker.createContainer({
                 name: containerName,
                 Image: DOCKER_TUNNEL_IMAGE,