Remove unused libraries and files from Docker image

.dockerignore

@@ -3,6 +3,7 @@
 .sass-cache
 log
 config/database.yml
+doc
 public/assets
 public/system
 tmp/*

Dockerfile

@@ -1,11 +1,7 @@
 FROM ruby:2.3
 
 RUN apt-get update && \
-    apt-get install --no-install-recommends -y \
-      cron \
-      libv8-dev \
-      libmagic-dev \
-      libmysqlclient-dev && \
+    apt-get install --no-install-recommends -y cron && \
     rm -rf /var/lib/apt/lists/* && \
     apt-get clean
 
@@ -13,8 +9,14 @@ ENV RAILS_ENV=production
 
 WORKDIR /usr/src/app
 COPY . ./
-RUN echo $SOURCE_COMMIT > REVISION && \
-    bundle install --without development --without test && \
+RUN echo $SOURCE_COMMIT > REVISION
+
+RUN buildDeps='libmagic-dev' && \
+    apt-get update && \
+    apt-get install --no-install-recommends -y $buildDeps && \
+    rm -rf /var/lib/apt/lists/* && \
+    bundle install --deployment --without development test && \
+    apt-get purge -y --auto-remove $buildDeps && \
     whenever --update-crontab
 
 EXPOSE 3000