xfce-build: Add an Xfce 4.14 image

I did a local test build, which was successful. Not sure if we want to really build this image periodically or if we just build it once and then push it to dockerhub for reference...

xfce/xfce-build:4.14 contains:
/git/xfce4-dev-tools: xfce-4.14.0
/git/libxfce4util: xfce-4.14.0
/git/xfconf: xfconf-4.14.4
/git/libxfce4ui: xfce-4.14.0
/git/exo: xfce-4.14.0
/git/garcon: xfce-4.14.0
/git/xfce4-panel: xfce4-panel-4.14.4
/git/thunar: thunar-1.8.16