Make it easier to build the docs

Following the instructions will install the latest version of Jekyll,
but the Brooklyn website is not compatible with this. Provide a
`Gemfile` and `Gemfile.lock` with known good versions of Jekyll and
other dependencies, and update the instructions to use `bundler`.
Change from @andreaturli for Docker port forwarding
Merge pull request #1420 from grkvlt/fix/docker

Change from @andreaturli for Docker port forwarding
Merge pull request #1394 from richardcloudsoft/doc-build-fixes

Make it easier to build the docs