Dockerfile: simplify theme build process, ensure consistent environment with bundle exec

Dockerfile

@@ -46,12 +46,11 @@ RUN apt-get update && apt-get install -y \
 
 FROM image_resize AS hci_theme 
 
-## Start prebuilding
-COPY package.json /
-RUN npm install --only=dev
+## Build Theme from source
+## Installs the built gem system-wide
 COPY . /
-RUN gem install bundler
-RUN bundle install
-RUN rake
-RUN rm -rf node_modules
-RUN rm package-lock.json
+RUN bundle install && \
+    bundle exec rake gem && \
+    gem install hci-theme.gem && \
+    rm -rf node_modules && \
+    rm package-lock.json