Release candidate image release for jruby 10

[HoneyryderChuck]

This would help use it in docker based CI environments. Cruby usually releases rc images before Christmas day.

[headius]

I have duplicated the Dockerfile from the other releases for 10-snapshot based on the latest build and it appears to work properly:

[] docker-jruby $ cd 10/jdk21     
[] jdk21 $ docker build -t jruby-10-jdk-21 .
[+] Building 41.6s (11/11) FINISHED
...
[] jdk21 $ docker run -it jruby-10-jdk-21
irb(main):001> JRUBY_VERSION
=> "10.0.0.0-SNAPSHOT"

I'm not sure how to safely publish the images somewhere without having them be a real release image, though...

[HoneyryderChuck]

I think it has to be a real release image. In the case of CRuby, that's tied to a preview/release-candidate version (example). Not sure whether this was your plan though.

I could also use latest or nightly instead to test jruby 10, just not sure whether this version is in the master branch and built regularly. Whatever works best for you.