Make QGIS Server service working

[justb4]

There are several problems with using QGIS Server service:

• using a 14-month old Docker Image (latest is obsolete, now called master)
• recent versions do not work as endpoint

See details at this issue:
camptocamp/docker-qgis-server#189

This started as an SSL-issue for HTML display (http requests blocked as for mixed content) but testing with recent Image versions revealed even more blocking errors.