Add docker support

[GabLeRoux]

My local ruby is always broken with things like

Your Ruby version is 2.6.3, but your Gemfile specified 2.4.2

Even if

rbenv local

2.4.2

Or things like

bundle install

Traceback (most recent call last):
	1: from /Users/gableroux/.gem/bin/bundle:23:in `<main>'
/Users/gableroux/.gem/bin/bundle:23:in `load': cannot load such file -- /usr/local/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/exe/bundle (LoadError)

So screw my local environment, let's docker all the things. Updated readme, fewer commands, more fun.

docker-compose ps

      Name                     Command               State           Ports
-----------------------------------------------------------------------------------
saglacio_jekyll_1   bundle exec jekyll server  ...   Up      0.0.0.0:4000->4000/tcp

docker-compose logs -f

 Configuration file: /app/_config.yml
             Source: /app
        Destination: /app/_site
  Incremental build: disabled. Enable with --incremental
       Generating...
                     done in 3.165 seconds.
  Auto-regeneration: enabled for '/app'
     Server address: http://0.0.0.0:4000
   Server running... press ctrl-c to stop.
       Regenerating: 1 file(s) changed at 2019-08-20 12:57:07 ...done in 3.9697251 seconds.
 Configuration file: /app/_config.yml
             Source: /app
        Destination: /app/_site
  Incremental build: disabled. Enable with --incremental
       Generating...
                     done in 2.304 seconds.
  Auto-regeneration: enabled for '/app'
     Server address: http://0.0.0.0:4000
   Server running... press ctrl-c to stop.

Works fine 👍