The reason Qt widgetset was not enabled: high build times (24-25 min). Other targets take 4-5 min each and updating build environment image process takes 19 min.
1. Added Dockerfile.buildenv to generate build environment image. It is based on Debian 11 x86_64 (contains GTK2, GTK3, Qt5 libraries and FPC (versions 3.2.0, 3.2.2) for x86_64-linux with x86_64-win64 and i386-win32 cross-compilers.
2. New CI configuration with jobs to build Lazarus with both FPC versions for x86_64-gtk2, x86_64-qt5, x86_64-win64, i386-win32. Also there is a job to update build environment image, which should be only run when Dockerfile.buildenv and/or FPC version constants in .gitlab-ci.yml file were updated.
All jobs were set to manual (don't trigger automatically) mode for now.
