Add ruby for use gem and install bundler for use bundle exec fastlane (#63)

Add ruby for use gem and install bundler for use bundle exec fastlane :)

Author: leolios

Dockerfile

@@ -29,6 +29,7 @@ RUN apt update -qq && apt install -qq -y --no-install-recommends \
         openjdk-8-jre \
         gnupg2 \
         python \
+        ruby-full \
         openssh-client \
         unzip \
     && rm -rf /var/lib/apt/lists/*;