Add a Docker-based setup.

I can't run Ruby 2.7.x on my machine anymore. Docker was the only
solution I found. It may help others getting started to contribute
to this website as well.

The setup is basically copied from that of `scala-lang.org`, but
with an additional setup of Node.js.

Author: sjrd

Dockerfile

@@ -0,0 +1,16 @@
+FROM ruby:2.7.5
+
+# We need Node.js for the `execjs` gem, used by `_plugins/tokenize.rb`
+RUN apt install -y curl
+RUN curl -fsSL https://deb.nodesource.com/setup_18.x | bash -
+RUN apt install -y nodejs
+
+RUN gem install bundler:2.3.10
+
+WORKDIR /srv/jekyll
+
+COPY Gemfile .
+COPY Gemfile.lock .
+
+RUN chmod u+s /bin/chown
+RUN bundle install

README.md

@@ -12,6 +12,35 @@ The key to contributing is being able to edit and
 preview your content. [Your pull requests are welcome](https://github.com/scala-js/scala-js-website/compare)!
 
 ## Set up
+
+### With Docker
+
+You need to have [Docker Engine](https://docs.docker.com/engine/) and [Docker Compose](https://docs.docker.com/compose/) installed on your machine.
+Under Mac OS (Intel or Apple silicon), instead of installing [Docker Desktop](https://docs.docker.com/desktop/) you can also use [HomeBrew](https://brew.sh/) with [Colima](https://github.com/abiosoft/colima): `brew install colima docker docker-compose`.
+UID and GID environment variables are needed to avoid docker from writing files as root in your directory.
+
+```
+env UID="$(id -u)" GID="$(id -g)" docker-compose up
+```
+
+On Linux you may have to prefix that command with `sudo`, depending on your Docker setup.
+
+The generated site is available at `http://localhost:4000`.
+
+When the website dependencies change (the content of the `Gemfile`), you have to re-build the Docker image:
+
+```
+env UID="$(id -u)" GID="$(id -g)" docker-compose up --build
+```
+
+If you have problems with the Docker image or want to force the rebuild of the Docker image:
+
+```
+env UID="$(id -u)" GID="$(id -g)" docker-compose build --no-cache
+```
+
+### Manually with Ruby tooling
+
 As this website is built with [Jekyll](http://jekyllrb.com/),
 we will need to set up some Ruby tooling.
 
@@ -22,7 +51,7 @@ $ rvm use 2.7.5 --install
 
 # Set up Bundler, a Ruby package manager
 # It downloads dependencies specified in a Gemfile
-# but into a local path unlike gem 
+# but into a local path unlike gem
 $ gem install bundler
  # and if this fails, try installing libffi first (distro-specific):
  # sudo apt install libffi-dev
@@ -34,8 +63,9 @@ $ bundle install
 $ bundle exec jekyll build
 ```
 
-## Editing live
-This is what you would do after initial installation:
+#### Editing live
+
+This is what you would do after the initial installation:
 ```bash
 $ bundle exec jekyll serve --watch
 ```

docker-compose.yml

@@ -0,0 +1,11 @@
+version: '2'
+
+services:
+  jekyll:
+    user: "${UID}:${GID}"
+    build: .
+    command: sh -c "chown $UID / && bundle exec jekyll serve --watch --host=0.0.0.0 "
+    ports:
+      - '4000:4000'
+    volumes:
+      - .:/srv/jekyll