Investigate toolchain method of creating Docker images

The platform team has come up with a different way of creating Docker images for Qt builds:

https://intranet.qt.io/display/QTRD/PoC%3A+using+boot2qt+toolchain+from+Docker+container

This method could potentially help with making slimmer Docker images, as well as better control of versions that end up in the image and thus their reproducibility.