anchor.diff:  
   # Install and hold a specific version of RabbitMQ
   RUN \
  -    apt-get -y --no-install-recommends install "rabbitmq-server=3.8.9-1" && \
  +    apt-get -y --no-install-recommends install "rabbitmq-server=3.8.12-1" && \
       apt-mark hold rabbitmq-server && \
       rm -rf /etc/rabbitmq/ && \
       true
  
positive.code: 
  # Add RabbitMQ APT repository
  RUN \
      curl -fsSL https://github.com/rabbitmq/signing-keys/releases/download/2.0/rabbitmq-release-signing-key.asc | apt-key add - && \
      echo "deb https://dl.bintray.com/rabbitmq-erlang/debian focal erlang-23.x" \
          > /etc/apt/sources.list.d/rabbitmq-erlang.list && \
      # At the time of writing, docs tell us to use "bionic" for 20.04:
      echo "deb https://dl.bintray.com/rabbitmq/debian bionic main" \
          > /etc/apt/sources.list.d/rabbitmq.list && \
      apt-get -y update && \
      true
  
  
negative.code: # and check "apt-cache policy erlang-nox | rabbitmq-server" for available
  # versions)
  RUN \
      export ERLANG_APT_PACKAGE_VERSION="1:23.2.3-1" && \
      apt-get -y --no-install-recommends install \
          "erlang-asn1=$ERLANG_APT_PACKAGE_VERSION" \
          "erlang-base=$ERLANG_APT_PACKAGE_VERSION" \
  