Add dockerfile for nexus
Cleaner docker instance for v2.9.0 of nexus
Remove config file mangling in preference to environment variables
Update nexus to 2.9.1
Fix incorrect path in README
Use latest stable instead of testing

Note that this is a rolling tag which means we won't have to update
it when stable changes.
Update to nexus 2.10.0-02
Add extension to README file
Update nexus url to new sonatype domain
Add sonatype to nexus reference