Properly determine host's source directory in local dev mode

src/Handler.php

@@ -341,6 +341,7 @@ class Handler extends BaseHandler {
    */
   private function getDockerMountSource($projectRoot): string {
     try {
+      $currentDir = getcwd();
       $output = [];
       exec('basename "$(cat /proc/1/cpuset)"', $output);
       $id = reset($output);
@@ -348,8 +349,15 @@ class Handler extends BaseHandler {
       exec('docker container inspect ' . $id, $output);
       $container = json_decode(implode('', $output), TRUE)[0];
       foreach ($container['Mounts'] as $mount) {
-        if (strpos($projectRoot, $mount['Destination']) === 0) {
-          return $mount['Source'] . substr($projectRoot, strlen($mount['Destination']));
+        if (empty($projectRoot)) {
+          if ($currentDir === $mount['Destination']) {
+            return $mount['Source'];
+          }
+        }
+        else {
+          if (strpos($projectRoot, $mount['Destination']) === 0) {
+            return $mount['Source'] . substr($projectRoot, strlen($mount['Destination']));
+          }
         }
       }
     }