title: What is "/app" working directory for a Dockerfile?
body: <p>In the <a href="https://docs.docker.com/get-started/part2/" rel="noreferrer">docker docs getting started tutorial part 2</a>, it has one make a Dockerfile.  It instructs to add the following lines:</p>
  
  <pre><code># Set the working directory to /app
  WORKDIR /app
  
  # Copy the current directory contents into the container at /app
  COPY . /app
  </code></pre>
  
  <p>What is /app, and why is this a necessary step?</p>