Don't advise people to 'use Docker to run coala' on the frontpage

[underyx]

We're not really maintaining the docker image (see inactivity of https://github.com/coala/docker-coala-base/issues/125). People will unknowingly use months old versions of coala if they do what we tell them to on the homepage, and that's not very nice.

[sils]

the docker release should happen ASAP, it's critical. It's a shame this didn't happen yet, people are going to use the docker image for CI anyway so I'd stay with it on the frontpage. Maybe we need to make releasing the docker image with a new version easier.