Dockerfile for bundle-audit

Dockerfile

FROM ruby:2.4.1

RUN gem install 'bundle-audit'

ADD Gemfile ./
ADD Gemfile.lock ./

# We never want the `bundle audit update` to be cached.  So this will help bust
# the cache if you include this in your build command:
# `--build-arg CACHEBUST=$(date +%s)`
# https://github.com/moby/moby/issues/1996#issuecomment-295683518
ARG CACHEBUST=1

RUN bundle audit update

CMD ["bundle-audit"]