Image for QGIS-Server 3.2

[hbruch]

This PR adds a 3.2 docker build. I changed the base image from kartoza/qgis-desktop to qgis/qgis (and added a build-arg for the base images docker_tag) to make it easier to switch to new base version. However, there might be reasons why you decided to use your proper base image instead that I'm not aware of(?).

This PR also fixes some issues with the current 3.0 branch (usr/lib/lib, switch to /var/www/.local intead of /var/www/.qgis2 folder, making io timeout and cachesize configurable via ENV).

EDIT: Ok, I see. qgis/qgis adds >5GB to the build-deps image, as docker layers are not optimized. However, the former kartoza/qgis-desktop is not compiled with the QGIS_SERVER=ON flag. Not sure what are the consequences.

EDIT2: I switched base image to qgis/qgis-build-deps, reused the build scripts from kartoza/qgis-desktop and adapted the build flags to match the current qgis/.docker/qgis.dockerfile flags. Especially, I enabled WITH_SERVER and disabled WITH_DESKTOP (which by-the-way solved a resource_rc.py build issue).