title: "Empty continuation lines will become errors"… how should I comment my Dockerfile now?
body: <p>Consider the following Dockerfile:</p>
  
  <pre class="lang-docker prettyprint-override"><code>FROM alpine:edge
  
  EXPOSE \
  # web portal
  8080 \
  # backdoor
  8081
  </code></pre>
  
  <p>Built like so:</p>
  
  <pre class="lang-bash prettyprint-override"><code>docker build .
  </code></pre>
  
  <p>We observe such output:</p>
  
  <pre><code>Sending build context to Docker daemon  17.1TB
  Step 1/2 : FROM alpine:edge
   ---&gt; 7463224280b0
  Step 2/2 : EXPOSE 8080 8081
   ---&gt; Using cache
   ---&gt; 7953f8df04d9
  [WARNING]: Empty continuation line found in:
      EXPOSE 8080 8081
  [WARNING]: Empty continuation lines will become errors in a future release.
  Successfully built 7953f8df04d9
  </code></pre>
  
  <p>So, given that it'll soon become illegal to put comments in the middle of a multi-line section: <strong>what's the new recommended way to comment multi-line commands</strong>?</p>
  
  <p>This is particularly important for <code>RUN</code> commands, since we are encouraged to reduce image layers by <code>&amp;&amp;</code>ing commands together.</p>
  
  <hr>
  
  <p>Not sure exactly when this was introduced, but I'm currently experiencing this in version:</p>
  
  <pre><code> docker --version
  Docker version 17.07.0-ce, build 8784753
  </code></pre>
  
  <p>I'm using Docker's <em>edge</em> release stream, so maybe this will not yet look familiar if you are using Docker stable.</p>