Union mounts into Docker containers
<p>If I have a directory d1/, I know I can mount it at /mountPoint inside a Docker container by doing this:</p>

<pre><code>docker run -v /path/to/d1:/mountPoint ...
</code></pre>

<p>However, I have two directories d1/ and d2/ (let's say they contain files a.txt and b.txt respectively).</p>

<p>I want to mount the union of these two directories at /mountPoint in my container, i.e. I want /mountPoint/a.txt and /mountPoint/b.txt to exist.</p>

<p>Given that Docker uses UnionFS a lot internally, I am rather hoping there are options to do a union mount at a specific path inside a container, but I can't find them if so.</p>