title: docker build is very slow even with simple commands
body: <p>I'm building a docker image on my Raspberry Pi, which is of course takes some time. The problem here is that even very simple commands in the Dockerfile like setting an environment variable, using <code>chmod +x</code> on a single file or exposing port 80 take minutes to complete.</p>
  
  <p>Here is an excerpt of my Dockerfile:</p>
  
  <pre><code>FROM resin/rpi-raspbian
  MAINTAINER felixbr &lt;mymail@redacted.com&gt;
  
  RUN export DEBIAN_FRONTEND=noninteractive &amp;&amp; apt-get update &amp;&amp; apt-get install -y python python-dev python-pip python-numpy python-scipy python-mysqldb mysql-server redis-server nginx dos2unix poppler-utils
  
  COPY requirements.txt /app/
  
  RUN pip install -r /app/requirements.txt
  
  COPY . /app
  
  WORKDIR /app
  
  RUN cp /app/nginx-django.cfg /etc/nginx/sites-enabled/default
  RUN chmod +x /app/start.sh
  
  ENV DOCKERIZED="true"
  
  CMD ./start.sh
  
  EXPOSE 80
  </code></pre>
  
  <p>Keep in mind this is using an <code>ARMv6</code> base image, so it can run on a Raspberry Pi and I'm using docker 1.5.0 built for the hypriot Raspberry Pi OS.</p>
  
  <p>Is it copying the built layers for every command or why does each of the last few commands take minutes to complete?</p>