Another Ruby version, another curveball. Node is no longer in the default package repository for Debian, so you have to manually add it. Here's how I got my Dockerfile to run:

# Dockerfile
FROM ruby:2.5.0
RUN apt-get update
RUN curl -sL https://deb.nodesource.com/setup_9.x | bash -
RUN apt-get install -y nodejs
RUN apt-get install -y npm
RUN npm install yarn -g

CMD mkdir /app
WORKDIR /app
COPY Gemfile /app
COPY Gemfile.lock /app

RUN gem install bundler -v 1.16.1
RUN bundle install