Utils: Introduce FilePath::fileSize()

And implement it locally and for docker.

Change-Id: I362f4cae95560836d9300473a286429eb4569329
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/filepath.h

@@ -139,6 +139,7 @@ public:
     bool removeRecursively(QString *error = nullptr) const;
     bool copyFile(const FilePath &target) const;
     bool renameFile(const FilePath &target) const;
+    qint64 fileSize() const;
 
     Qt::CaseSensitivity caseSensitivity() const;