appimage: Add xcb-util-devel package to docker

* docker/appimage/Dockerfile: Add xcb-util-devel package.

Qt 5.15.2 searches for xcb-util by looking for xcb/xcb-util.h
header. This header is provided by the xcb-util-devel which
sadly pulls in a lot of other dependencies. This enlarges
the docker image quite a bit.