Updated Docker setup.

* Use ruby version 3
* Use bundle container
* Run rails commands as app user

Dockerfile

@@ -1,39 +1,38 @@
-FROM ruby:2.1-slim
+# Based on ruby image, packed with a lot of development dependencies
+FROM ruby:2.3
 
+# Install all dependencies for development and testing
 RUN apt-get update && \
     apt-get install --no-install-recommends -y \
       mysql-client \
-      git \
-      make \
-      gcc \
-      g++ \
-      patch \
-      libsqlite3-dev \
       libv8-dev \
       libmysqlclient-dev \
-      libxml2-dev \
-      libxslt1-dev \
-      libffi-dev \
-      libreadline-dev \
       xvfb \
       iceweasel && \
     rm -rf /var/lib/apt/lists/* && \
     apt-get clean
 
-ENV WORKDIR /usr/src/app
+# Run app and all commands as user 'app'. This avoids changing permissions
+# for files in mounted volume.
+RUN adduser --gecos GECOS --disabled-password --shell /bin/bash app
+USER app
 
-RUN mkdir -p $WORKDIR
-WORKDIR $WORKDIR
+# Create an directory to store the application code.
+RUN mkdir /home/app/src
+WORKDIR /home/app/src
 
 # Copy plugins before the rest to allow bundler loading gemspecs
 # TODO: Move plugins to gems and add them to Gemfile instead
-COPY plugins $WORKDIR/plugins
+COPY plugins ./plugins
 
-COPY Gemfile $WORKDIR/
-COPY Gemfile.lock $WORKDIR/
-RUN bundle install --jobs 4
+# Add Gemfiles and run bundle.
+COPY Gemfile Gemfile.lock ./
+ENV BUNDLE_JOBS=4 BUNDLE_PATH=/home/app/bundle \
+    BUNDLE_APP_CONFIG=/home/app/bundle/config
+RUN bundle install
 
-COPY . $WORKDIR
+# Copy the application code. (Excluded files see .dockerignore)
+COPY . ./
 
 EXPOSE 3000