(maint) Dockerfile change how dumb-init downloaded
(maint) Dockerfile change how dumb-init downloaded

 - Instead of using wget in a run command, use Dockerfile ADD to grab a
   file from a URI. This should integrate better with the builders
   caching behavior.

 - This also moves a needless ENV var out to a build-arg with a default
   value, which makes more sense.

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