add metadata to Dockerfile

Dockerfile

 FROM ruby:2.3.1-alpine
 
+LABEL maintainer="https://github.com/tootsuite/mastodon" \
+      description="A GNU Social-compatible microblogging server"
+
 ENV RAILS_ENV=production \
     NODE_ENV=production
 
+EXPOSE 3000 4000
+
 WORKDIR /mastodon
 
 COPY Gemfile Gemfile.lock package.json yarn.lock /mastodon/