feat: renovatebot support for ruby projects

Install ruby and bundler on the base image to properly generate
project lockfiles.

Dockerfile.renovate

@@ -22,7 +22,7 @@ RUN apt-get install -yq \
         git jq curl bash build-essential libssl-dev zlib1g-dev \
         libbz2-dev libreadline-dev libsqlite3-dev wget unzip \
         ca-certificates openssh-client lsb-release python3-dev \
-        python3-pip
+        python3-pip ruby bundler
 # Install nodejs: required for renovate
 RUN curl -sL https://deb.nodesource.com/setup_16.x | bash - \