title: datadog agent not reachable from inside docker container
body: <p>I installed dd-agent on Amazon linux ec2. If I run my python script directly on the host machine (I used the SDK named "dogstatsd-python"), all the metrics can be sent to datadog (I logged in to datadoghq.com and saw the metrics there). 
  the script is something like:</p>
  
  <pre><code>from statsd import statsd
  statsd.connect('localhost', 8125)
  statsd.increment('mymetrics')
  </code></pre>
  
  <p>However, I launched a docker container and run the same script from inside the container:</p>
  
  <pre><code>from statsd import statsd
  statsd.connect('172.14.0.1', 8125)
  statsd.increment('my metrics')
  </code></pre>
  
  <p>'172.14.0.1' is the IP of the host, which was extracted with command </p>
  
  <pre><code>netstat -nr | grep '^0\.0\.0\.0' | awk '{print $2}'
  </code></pre>
  
  <p>No metrics were sent to datadog at all.....</p>
  
  <p>I'm guessing that maybe it's due to some configuration issue like "address binding". Maybe the dd-agent I installed on the host can only receive metrics from 'localhost'. </p>
  
  <p>Hope someone could help me. Thank you in advance. </p>