Docker: Don't try to mount existing directories

Change-Id: I1df4e1bf8689edf08c539d71529e4149909502d7
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/docker/dockerdevice.cpp

@@ -1208,6 +1208,9 @@ bool DockerDevicePrivate::ensureReachable(const FilePath &other)
             return true;
     }
 
+    if (q->filePath(other.path()).exists())
+        return false;
+
     addTemporaryMount(other, other);
     return true;
 }