anchor.diff:  
   # Install Python requirements
   COPY requirements.txt ./  
  -RUN pip install --no-cache-dir -r requirements.txt
  +RUN pip3 install --no-cache-dir -r requirements.txt
   
   # Copy Python app files
   COPY . ./
  
positive.code: 
  # Run Celery as the user `worker`
  USER worker
  CMD celery worker --app worker --queues stencila --hostname worker@stencila
  
negative.code: FROM stencila/executa:20200414.1
  
  # This is necessary because the executa image
  # defaults to a guest user.
  USER root
  
  # Create `worker` user and use it's home dir
  RUN useradd -m worker
  