Fix Docker setup

[julienrf]

The Docker setup was not working for me, most probably because it was not using a specific version of the jekyll image, and maybe the latest images have broken something?

This PR introduces our own Dockerfile, which uses the same version of ruby as on the CI (2.5), and installs Jekyll from there. When this Docker image is built, the website dependencies are fetched and cached (ie, we run bundle install during the Docker image build time).

[julienrf]

@vincenzobaz I’ve simplified the Dockerfile according to your advice, and I’ve also updated the README and removed unused (I think) files.

[adpi2]

Thanks @julienrf it's been easier for me to make the site work with this docker setup.

It seems the docker-compose config in https://github.com/scala/docs.scala-lang is different. So which one is best?

[julienrf]

It seems the docker-compose config in https://github.com/scala/docs.scala-lang is different. So which one is best?

Good point. Vincenzo also raised this point. We should probably try to unify them. On my side, my experience with docs.scala-lang.org was that I was unable to get incremental rendering work properly (basically it takes 1min30 to re-generate the website every time I change something, whereas in this PR the scala-lang.org website takes a couple of seconds to re-generate). What is your experience?

[adpi2]

I have the same experience than you with docs.scala-lang: incremental build does not seem to work at all. It would be way better with the setup of this PR.