Adding a docker-compose template for running Mastodon easily

Dockerfile

@@ -0,0 +1,15 @@
+FROM ruby:2.2.4
+
+ENV RAILS_ENV=production
+
+RUN apt-get update -qq && apt-get install -y build-essential libpq-dev
+RUN mkdir /mastodon
+
+WORKDIR /mastodon
+
+ADD Gemfile /mastodon/Gemfile
+ADD Gemfile.lock /mastodon/Gemfile.lock
+
+RUN bundle install --deployment --without test --without development
+
+ADD . /mastodon