Comment 2 for bug 1784193

In my bionic container, I installed clamav-daemon (and its dependencies) and it worked right out of the box.

Of note, however, is this extend.conf systemd file shown in systemctl status clamav-daemon:
root@bionic-clamav-1784193:~# systemctl status clamav-daemon
● clamav-daemon.service - Clam AntiVirus userspace daemon
   Loaded: loaded (/lib/systemd/system/clamav-daemon.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/system/clamav-daemon.service.d
           └─extend.conf
   Active: inactive (dead)
     Docs: man:clamd(8)
           man:clamd.conf(5)
           https://www.clamav.net/documents/

That file has the mkdir and chown commands you showed:
root@bionic-clamav-1784193:~# cat /etc/systemd/system/clamav-daemon.service.d/extend.conf
[Service]
ExecStartPre=-/bin/mkdir /run/clamav
ExecStartPre=/bin/chown clamav /run/clamav

That is created by clamav-daemon's postinst script, so it should have happened when you installed the package, unless that failed for some reason (not shown in this bug report).

These are the packages I have:
ii clamav 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - command-line interface
ii clamav-base 0.100.1+dfsg-1ubuntu0.18.04.2 all anti-virus utility for Unix - base package
ii clamav-daemon 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - scanner daemon
ii clamav-freshclam 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - virus database update utility
ii libclamav7:amd64 0.100.1+dfsg-1ubuntu0.18.04.2 amd64 anti-virus utility for Unix - library

Can you try removing your workaround, and reinstalling the packages? And check if that extend.conf file is created, and/or if there is some error during postinst?

The only thing I saw when installing it was a harmless id command showing that the clamav user doesn't exist at the time of installation:
...
Setting up clamav-base (0.100.1+dfsg-1ubuntu0.18.04.2) ...
id: ‘clamav’: no such user
Processing triggers for ureadahead (0.100.0-20) ...
...