ProjectExplorer: Allow devices to report tasks in validate

And use it to report a hard error when docker devices don't have
a shared directory set. While this is possibly too harsh in the
long run it fits the current situation where we want to use the
container for building projects.

Change-Id: I1430716b5722fba537bb964463ce25c7001be6a4
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/docker/dockerdevice.h

@@ -62,6 +62,7 @@ public:
     static Ptr create(const DockerDeviceData &data) { return Ptr(new DockerDevice(data)); }
 
     ProjectExplorer::IDeviceWidget *createWidget() override;
+    QVector<ProjectExplorer::Task> validate() const override;
 
     bool canCreateProcess() const override { return true; }
     ProjectExplorer::DeviceProcess *createProcess(QObject *parent) const override;