title: Determine OS type in docker
body: <p>I want to download an sdk depending on the OS type my docker image is running on. How can I write the below pseudo code in docker script</p>
  
  <pre><code>RUN variable x = getOS()
  if [ "$x" = "Darwin" ]; then
       RUN wget -q http://xxx/android-ndk-xxxx-darwin-x86_64.bin
  else
       RUN wget -q http://xxx/android-ndk-xxxx-linux-x86_64.bin
  </code></pre>