RFC: Dockerfile based on Qt 5.12 and clang

Summary:
Instead of bringing the Qt binaries it builds our own.
It also finally solves some annoyances like including openssl by default.

Test Plan: I plan to test it, haven't tested yet.

Differential Revision: https://phabricator.kde.org/D18173
2 jobs for master
Status Job ID Name Coverage
  Build
manual #1177
docker allowed to fail manual
kdeorg/appimage-base
manual #1178
docker allowed to fail manual
kdeorg/staticweb