title: How to check whether python package is installed or not in Docker?
body: <p>I used Dockerfile successfully built a container. However, my code doesn't work in the container. It does work if I install all the packages manually. I'm assuming I messed up something that cause docker didn't install the packages properly. So, I want to check whether python package is installed or not in Docker container. What is the best way to check it?</p>
  
  <p>The Dockerfile I used:</p>
  
  <pre><code># Update the sources list
  RUN sudo apt-get update
  
  # Install basic applications
  RUN sudo apt-get install -y tar git curl nano wget dialog net-tools build-essential
  
  # First install ZeroMQ
  RUN sudo apt-get install -y libzmq-dev
  
  # Install libevent
  RUN sudo apt-get install -y libevent-dev
  
  # Install Python and Basic Python Tools
  RUN sudo apt-get install -y python python-dev python-setuptools
  RUN sudo apt-get install -y python-pip 
  
  # Add the current directory to the container
  ADD . /root/code
  
  # Get pip to download and install requirements:
  RUN sudo pip install -r /root/code/requirements.txt
  
  # Expose ports
  EXPOSE 80 4242
  
  # Define working directory.
  WORKDIR /root/code
  
  # Start the tcp server.
  CMD python app.py
  </code></pre>
  
  <p>The requirements.txt I used:</p>
  
  <pre><code>gevent==1.0.1
  greenlet==0.4.5
  msgpack-python==0.4.2
  pyzmq==13.1.0
  wsgiref==0.1.2
  zerorpc==0.4.4
  </code></pre>