Docker support for production use

This commit adds a Docker infrastructure that is ready for production
use. It is meant to simplify the deployment of OSEM for everyone who
wants to host their own instances.

It includes many features like data persistence, automatic secret key
generation and persistence and automatic database initialization and
upgrading. This should make updating the Docker container as easy as
possible.

Dockerfile

@@ -0,0 +1,46 @@
+FROM ruby:2.3
+
+MAINTAINER TheAssassin <theassassin@users.noreply.github.com>
+
+# required for compiling assets
+RUN apt-get update && \
+    apt-get install -y nodejs nodejs-legacy mariadb-client
+
+# used to run the container without root permissions
+RUN adduser --home /osem/ --system --group --disabled-login --disabled-password osem
+
+# required to detect when the database is up and running in init.sh
+RUN cd /usr/bin && \
+    wget https://github.com/jwilder/dockerize/releases/download/v0.3.0/dockerize-linux-amd64-v0.3.0.tar.gz -O dockerize.tar.gz && \
+    echo "36e8319cdf9d2b07340f456ec61cfa0f495ec6c130b02ad9c116fd55a5c43fa1  dockerize.tar.gz" | sha256sum -c && \
+    tar -xf dockerize.tar.gz && \
+    rm dockerize.tar.gz
+
+# explicitly add Gemfile and install dependencies using bundler to make use of
+# Docker's caching
+WORKDIR /osem/
+RUN gem install puma
+COPY Gemfile /osem/
+COPY Gemfile.lock /osem/
+RUN bundle install --without test development
+
+# add OSEM files and prepare them for use inside a Docker container
+COPY . /osem/
+RUN chown osem.osem /osem/ -R && \
+    mv /osem/config/database.yml.docker /osem/config/database.yml
+
+# data directory is used to cache the secret key in a file
+ENV DATA_DIR /data
+RUN install -d -m 0700 -o osem $DATA_DIR
+VOLUME ["$DATA_DIR"]
+
+USER osem
+EXPOSE 9292
+
+COPY docker/init.sh /init.sh
+
+# a user could override this if they wanted to serve the static files directly
+# from a webserver
+ENV RAILS_SERVE_STATIC_FILES 1
+
+CMD ["bash", "/init.sh"]