anchor.diff:  # docker 1.11.0+ now relies on four binaries
   # See https://github.com/docker/docker/wiki/Engine-1.11.0
   # See https://docs.docker.com/engine/installation/binaries/
  -#
  -# After this, the cyber-dojo user can do
  -# $ sudo -u docker-runner sudo docker ...
   # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   
   ARG  DOCKER_VERSION
  
positive.code:   && apk del curl
  
  # - - - - - - - - - - - - - - - - - - - - - -
  # 5. install ruby and json gem
  
  USER root
  RUN  apk update && apk add ruby ruby-irb ruby-io-console ruby-bigdecimal tzdata bash
  RUN  gem install json_pure --no-ri --no-rdoc
  
  # - - - - - - - - - - - - - - - - - - - - - -
  # 6. install commander
  
  RUN  mkdir /app
  COPY cyber-dojo.sh          /app
  
negative.code:   && rm /docker-${DOCKER_VERSION}.tgz \
    && apk del curl
  
  # - - - - - - - - - - - - - - - - - - - - - -
  # 2. install docker-compose
  # https://github.com/marcosnils/compose/blob/master/Dockerfile.run
  
  ARG  DOCKER_COMPOSE_VERSION
  ARG  DOCKER_COMPOSE_BINARY=/usr/bin/docker-compose
  USER root
  RUN  apk update \
    && apk add --no-cache curl openssl ca-certificates \
  