build: Simplify gem command in Debian Docker image

Dockerfile.builder

@@ -6,4 +6,4 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 	locales rubygems ruby-dev build-essential git \
 	&& apt-get clean \
 	&& rm -rf /var/lib/apt/lists/* \
-	&& gem install --no-ri --no-rdoc fpm
+	&& gem install fpm