title: Docker ADD doesn't extract file
body: <p>I'm trying to download a specific Node version and copy the binary from the extracted tar.xz, but it doesn't look like <code>ADD</code> actually extracts the file.</p>
  
  <p>This is my Dockerfile:</p>
  
  <pre><code>FROM arm32v7/debian:stretch-slim
  
  ENV nodever="v6.10.0"
  ADD https://nodejs.org/dist/${nodever}/node-${nodever}-linux-armv7l.tar.xz /
  COPY /node-${nodever}-linux-armv7l/bin/node /
  
  RUN cat /etc/issue &amp;&amp; \
  \
  apk update &amp;&amp;  \
  apk add \
      dnsmasq \
      hostapd \
      iptables
  
  RUN mkdir app
  ADD start.sh /app/start.sh
  ADD server /app/server
  
  ADD configs/hostapd.conf /etc/hostapd.conf
  ADD configs/dnsmasq.conf /etc/dnsmasq.conf
  
  WORKDIR /app
  CMD ["/app/start.sh"]
  </code></pre>
  
  <p>but running <code>docker -v build</code> yields this error:</p>
  
  <pre><code>Sending build context to Docker daemon  9.728kB
  Step 1/12 : FROM arm32v7/debian:stretch-slim
  
  ---&gt; 1510cdc40eca
  Step 2/12 : ENV nodever "v6.10.0"
  
  ---&gt; Using cache
  ---&gt; 1ca3d2f72a3c
  Step 3/12 : ADD https://nodejs.org/dist/${nodever}/node-${nodever}-linux-armv7l.tar.xz /
  Downloading   8.34MB/8.34MB
  
  ---&gt; a1fefb9bda66
  Step 4/12 : COPY /node-${nodever}-linux-armv7l/bin/node /
  COPY failed: stat /var/lib/docker/tmp/docker-builder233292954/node-v6.10.0-linux-armv7l/bin/node: no such file or directory
  </code></pre>
  
  <p>I've tried running <code>RUN ls /</code> which shows that it has downloaded the archive but it isn't extracted.</p>