Docker: Switch Linux file handling to docker --exec by default

Also on Linux, where there's actually a choice.

It's slower, but less fragile when it comes to user ids
and/or file system permissions.

Change-Id: I811612fcff65fad50be18bed21150f6ca7fae93c
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/docker/dockerdevice.h

@@ -44,7 +44,7 @@ public:
     QString tag;
     QString size;
     bool useLocalUidGid = true;
-    bool useFilePathMapping = true;
+    bool useFilePathMapping = false;
     QStringList mounts;
 };