The Ruby Docker image was failing to install rubocop due to missing

development dependencies. We install these in the base image and then do

an initial install of RuboCop to get the right things in the right

place, and then derive a RuboCop install from that in the Ruby image.
The problem is, when we first set this up, we didn't need the

ruby-dev package to build it, but we do now. Something about the base

copy was installed fine, but this ultimately triggered a recompile in

the Ruby image when doing the follow-up install of the built copy, which

then failed.
We now simply include this in the base image we build from.
Along with this, there are some fixes for mixed indentation.

Successfully built the complete set of Docker images.