Use ubuntu 20.04 as base to ensure gdal version is >= 3.1
Use ubuntu 20.04 as base to ensure gdal version is >= 3.1

In ubuntu 18.04 the gdal version in ppa:ubuntugis/ubuntugis-unstable is only 3.0, but in 20.04 it is 3.2.1.

OpenDroneMap/WebODM,OpenDroneMap/WebODM,OpenDroneMap/WebODM,OpenDroneMap/WebODM,OpenDroneMap/WebODM