appimage: Use src/playground as build root

* src/appimage/Makefile.am: Use src/playgorund
as build root.
* src/appimage/build-appimage.sh: Update accordingly.

To enable rebuilding a tarball inplace and to be
more in line with the Windows build, use src/playground
as the build root.

The docker script mounts the gpg4win checkout under
/build and no longer uses a different /src dir.