Official Dockerfile Spec References Outdated XSpec and Saxon HE Runtime

[aj-stein-nist]

Thanks in advance for the wonderful project. Been using it off and on for a few years in previous jobs.

I want to use XSpec in an automation pipeline I noticed the reference build (from 0.5.x range) and the Saxon HE version are very old at this time. Since this is the canonical reference version, can we update it accordingly?