Added isolated build in docker

[aido93]

QTXDG_MINIMUM_VERSION changed to 3.4.0 (it is native to Ubuntu Focal) and added more accurate requirements to libraries. Binary appears in current directory after executing commands in Build section.

[tsujan]

I'm afraid QTXDG_MINIMUM_VERSION will be changed to 3.7.0 for the next release in April.

In addition, LXQt doesn't contain distro-specific codes.

[aido93]

3.7.0 is not a problem.

Could you update dependency list in readme.md? It has differences with CMakeLists.txt.

Also, I tried to build the application in different environments such as Alpine, debian 8, debian 7, etc with different gcc version (7, 8, 9, 10) and I didn't manage to do it.

I managed to build it in isolated environment only for ubuntu:focal.

[tsujan]

Could you update dependency list in readme.md? It has differences with CMakeLists.txt.

Yes, readme.md contains outdated info and needs to be updated. The reason may be that screengrab didn't belong to LXQt at first.