The X server writes a temporary file to
/usr/X11R6/lib/X11/xkb/compiled. Since we want to mount /usr
read-only (in accordance with the linux file system
standard, I guess), that directory should be link to
somewhere in the /var subtree.
This will be fixed in XFree86-220.127.116.11-26 and later.
/usr/X11R6/lib/X11/xkb/compiled is now symlinked to /var/lib/xkb.
Thank you for the report.