[thredds] The thredds-docker 5.9 release

[Jim Fluke <james.fluke@xxxxxxxxxxxxx>]

For Unidata/thredds-docker on GitHub, I see that there is a 5.9 release 
branch and a PR, and a new 5.9 image on DockerHub. Is there an ETA on 
when it will be merged into the main branch? Is it okay to upgrade to it 
before it gets merged?

Thanks,
Jim