v6.3.0

CHANGES:

• Add ?chown option for copy and add Dockerfile
  functions (#12 @talex5)
• add beta repository for switches if there is a dev release
  in any of the compilers for that switch (@avsm).
• Demote Debian 9 to a Tier 2 now that Debian 10 is stable (@avsm).
• Create opam group on all Linux distributions (#11 @talex5)