Load a database "gis6" with boundary data (not shown by default in the SomeoneElse-style style).
The "update_boundaries.sh" script is similar to "update_render.sh", except that it uses "osmium tags-filter" and "osm-tags-transform" to trim only "admin_level" data from an input .pbf file. It loads that into a database "gis6", which is made available as an overlay map layer.