"Warning: DocumentRoot [/var/tmp/catalyst/builds/default] does not exist" ― but it does

Question

I'm trying to use Apache to create a build server for hosting ISO images compiled with Gentoo Catalyst:

<VirtualHost *:80>
    ServerName localhost
    Include /etc/apache2/vhosts.d/default_vhost.include

    <IfModule mpm_peruser_module>
        ServerEnvironment apache apache
    </IfModule>

    DocumentRoot /var/tmp/catalyst/builds/default
    ServerAdmin realkstrawn93@realkstrawn93-miner.attlocal.net

    <Directory /var/tmp/catalyst/builds/default>
        Options +Indexes
        AllowOverride all
        Require all granted
    </Directory>

</VirtualHost>
</IfDefine>

When I go to attempt to access this, however, I get both the 403 error and the warning described in the title.

Here's the output of ls -l /var/tmp/catalyst/builds/default:

total 3397440
-rwxr-xr-x 1 apache apache 1701838848 Oct 17 22:23 livecd-amd64-installer-latest.iso
-rwxr-xr-x 1 apache apache       2756 Oct 17 22:23 livecd-amd64-installer-latest.iso.CONTENTS
-rwxr-xr-x 1 apache apache        736 Oct 17 22:24 livecd-amd64-installer-latest.iso.DIGESTS
-rwxr-xr-x 1 apache apache 1396285061 Oct 18 07:09 livecd-stage1-amd64-installer-latest.tar.bz2
-rwxr-xr-x 1 apache apache   26764352 Oct 18 07:10 livecd-stage1-amd64-installer-latest.tar.bz2.CONTENTS
-rwxr-xr-x 1 apache apache        780 Oct 18 07:10 livecd-stage1-amd64-installer-latest.tar.bz2.DIGESTS
drwxr-xr-x 1 apache apache        102 Oct 17 22:19 livecd-stage2-amd64-latest
-rwxr-xr-x 1 apache apache  354817796 Oct 15 04:29 stage3-amd64-latest.tar.xz

As you can see, the modes are definitely such that all files within that directory are readable, and I ran chown -R apache:apache /var/tmp/catalyst/builds/default multiple times. What else is necessary here?