title: Unable to locate package python
body: <p>I'm trying to run the below Dockerfile contents on ubuntu image.</p>
  
  <pre><code>FROM ubuntu
  RUN apt-get update
  RUN apt-get install -y python
  RUN apt-get install -y python-pip
  RUN pip install flask
  COPY app.py /opt/app.py
  ENTRYPOINT FLASK_APP=/opt/app.py flask run --host=0.0.0.0
  </code></pre>
  
  <p>But i'm getting the below error at layer 3.</p>
  
  <pre><code>Step 1/7 : FROM ubuntu
  ---&gt; 549b9b86cb8d
  Step 2/7 : RUN apt-get update
  ---&gt; Using cache
  ---&gt; 78d87d6d9188
  Step 3/7 : RUN apt-get install -y python
  ---&gt; Running in a256128fde51
  Reading package lists...
  Building dependency tree...
  Reading state information...
  E: Unable to locate package python
  </code></pre>
  
  <p>Athough while I run the below command individually</p>
  
  <p><code>sudo apt-get install -y python</code></p>
  
  <p>It's successfully getting installed.</p>
  
  <p>Can anyone please help me out.</p>
  
  <p>Note: I'm working behind organization proxy.</p>