title: Installing libraries in ubuntu image using Docker
body: <p>I am trying to install the 'sl' package in my ubuntu:14.04 image. Here's the content of my Dockerfile :</p>
  
  <pre><code>FROM ubuntu:14.04
  MAINTAINER xyz &lt;xyz@gmail.com&gt;
  RUN echo 'Going to install sl now'
  RUN apt-get install -y sl
  </code></pre>
  
  <p>And this is the command I'm using to build the image:</p>
  
  <pre><code>sudo docker build -t xyz/ubuntu:14.04 .
  </code></pre>
  
  <p>But I'm getting this error message:</p>
  
  <pre><code>Sending build context to Docker daemon 2.048 kB
  Sending build context to Docker daemon 
  Step 0 : FROM ubuntu:14.04
   ---&gt; d0955f21bf24
  Step 1 : MAINTAINER xyz &lt;xyz@gmail.com&gt;
   ---&gt; Using cache
   ---&gt; a6e08926e788
  Step 2 : RUN echo 'Going to install sl now'
   ---&gt; Using cache
   ---&gt; 1bf0bc6b3092
  Step 3 : RUN apt-get install -y sl
   ---&gt; Running in f7e57167443f
  Reading package lists...
  Building dependency tree...
  Reading state information...
  E: Unable to locate package sl
  INFO[0004] The command [/bin/sh -c apt-get install -y sl] returned a non-zero code: 100 
  </code></pre>
  
  <p>Where am I going wrong?</p>