Fix Docker setup

Dockerfile

@@ -0,0 +1,10 @@
+FROM ruby:2.5
+
+RUN gem install bundler jekyll
+
+WORKDIR /srv/jekyll
+
+COPY Gemfile .
+COPY Gemfile.lock .
+
+RUN bundle install

README.md

@@ -17,12 +17,20 @@ Markdown.
 
 ## Building the site
 Make sure you are in the root directory of the cloned repository.
-### For Compose:
+### With `docker-compose`:
 ```
-bin/serve
+docker-compose up
 ```
 
-### For Bundler:
+When the website dependencies change (the content of the `Gemfile`),
+you have to re-build the Docker image:
+
+```
+docker-compose up --build
+```
+
+### With Bundler:
+
 ```
 bundle install
 bundle exec jekyll serve --incremental

docker-compose.yml

@@ -1,12 +1,9 @@
 version: "2"
 services:
   scala-lang:
-    image: jekyll/jekyll:latest
-    volumes:
-      - $PWD:/srv/jekyll
-      - $PWD/vendor/bundle:/usr/local/bundle
+    build: .
+    command: bundle exec jekyll serve --incremental --host=0.0.0.0
     ports:
       - 4000:4000
-      - 35729:35729
-      - 3000:3000
-      - 80:4000
+    volumes:
+      - .:/srv/jekyll