Generate absolute urls for file layers

https://github.com/netzmacht/contao-leaflet-maps/issues/92

src/Bundle/Resources/config/mappers.yml

@@ -77,6 +77,7 @@ services:
     class: Netzmacht\Contao\Leaflet\Mapper\Layer\FileLayerMapper
     arguments:
       - '@netzmacht.contao_toolkit.repository_manager'
+      - '@netzmacht.contao_toolkit.contao.environment_adapter'
     tags:
       - { name: netzmacht.contao_leaflet.mapper }