Skip GDAL drivers to reduce build time

[dbaston]

We don't need widespread format support to test PostGIS.

For inspiration, see https://github.com/OSGeo/gdal/blob/master/gdal/docker/alpine-ultrasmall/Dockerfile#L113

[tobwen]

ultrasmall got discontinued. Alternatives: https://github.com/OSGeo/gdal/tree/master/docker