(maint) Minor Dockerfile refactor
(maint) Minor Dockerfile refactor

 - Extract same variables in the edge / release stages, so dupelicated
   RUN code can potentially be refactored into final stage

 - Hoist the UBUNTU_CODENAME to the top as an ARG rather than an ENV
   var which needn't be exposed in the container. Reuse that ARG in the
   edge and release stages

 - Instead of using wget inside a RUN block, use the ADD command to grab
   files from urls which is more cache friendly

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb