Added installation of varnish, apt-utils and rvm

[next-fernandocerezal]

It seems these tools are needed seeing the launching script and the logs of installation.

[chriswhong]

Hi, thanks for this. Did your build fail without these? I thought I had already included everything that was needed but I guess that is always subject to change because the Dockerfile is always pulling the latest code from Carto's repos.

[next-fernandocerezal]

Hello,
build was done with errors. The startup script uses starts varnish, but it was not installed, load rvm variables, but rvm was not installed, and apt-utils is installed as dependency of other packages later, but apt shows an error at the beginning. So I think they are needed.