Officially deprecate "rails"

rails/deprecated.md

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+This image is officially deprecated in favor of [the standard `ruby` image](https://hub.docker.com/_/ruby/), and will receive no further updates after 2016-12-31 (Dec 31, 2016). Please adjust your usage accordingly.
+
+For most usages of this image, it was already not bringing in `rails` from this image, but actually from your project's `Gemfile`, so the only "value" being added here was the pre-installing of `nodejs`, `mysql-client`, `postgresql-client`, and `sqlite3` for various uses of the `rails` framework.
+
+For example, a `Dockerfile` similar to the following would be a good starting point for a Rails project using PostgreSQL:
+
+```dockerfile
+FROM ruby:2.3
+
+RUN apt-get update && apt-get install -y postgresql-client --no-install-recommends && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /usr/src/app
+COPY Gemfile* ./
+RUN bundle install
+COPY . .
+
+EXPOSE 3000
+CMD ["rails", "server", "-b", "0.0.0.0"]
+```