Apache on Docker can't write to volume filesystem

I built an image for running Apache with mod_php, for Magento extension development. Magento needs to write to the webroot: It keeps files in /srv/magento/var/ for cache, error reporting, and some other features. The webroot on this image is a docker volume, and Apache doesn't run as root, so it can't write to the filesystem, so Magento fails.

I can't actually chown or chmod the directory within the container reliably. I don't want to use a Docker Volume Container because devs should have direct access to files in the Magento webroot. I wouldn't particularly mind running Apache as root in the container, but apachectl sure does seem to mind.

What's the appropriate way to give the Apache user in a Docker container write access to a volume?

Consider this example case:

$ cd $(mktemp -dt$(date +%s))
$ docker run -d -p 80:80 -v "$PWD:/srv/magento" kojiromike/magento_apache
$ cat > index.php <<PHP
<?php file_put_contents('foo', 'bar');
$ wget -SO/dev/null http://$(boot2docker ip 2>/dev/null)/index.php
--2014-12-15 13:33:59--
Connecting to connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  Date: Mon, 15 Dec 2014 17:18:49 GMT
  Server: Apache/2.2.22 (Debian)
  X-Powered-By: PHP/5.4.35-0+deb7u2
  Vary: Accept-Encoding
  Content-Length: 0
  Keep-Alive: timeout=5, max=100
  Connection: Keep-Alive
  Content-Type: text/html

    The file is already fully retrieved; nothing to do.

$ ls # Expecting 'foo' to exist
$ docker exec -ti $(docker ps -lq) tail -n 4 /var/log/apache2/error.log
[Mon Dec 15 17:18:49 2014] [error] [client] PHP Warning:  file_put_contents(foo): failed to open stream: Permission denied in /srv/magento/index.php on line 1
[Mon Dec 15 17:18:49 2014] [error] [client] PHP Stack trace:
[Mon Dec 15 17:18:49 2014] [error] [client] PHP   1. {main}() /srv/magento/index.php:0
[Mon Dec 15 17:18:49 2014] [error] [client] PHP   2. file_put_contents() /srv/magento/index.php:1

On VirtualBox, at least, it seems you can't change the ownership of a share. Since boot2docker uses VirtualBox for most of the developers I'm trying to support, I can't expect to solve my problem with chown. My problem wouldn't be difficult if I could use Docker Volume Containers, but that hampers developer use. So I've come up with another solution: I wrote a script that runs Apache as whatever user owns the webroot.

The meat of it is:

adduser --system --uid=$(stat -c %u .) "$owner"
echo "APACHE_RUN_USER=$owner" >> /etc/apache2/envvars

If you can't beat 'em, join 'em.