I'm seeing the same issue as @Barry Staes on a fresh install. The container installs fine but "/mnt/cache/appdata/clamav" (and /mnt/user/appdata/clamav) is created 755 with owner "nobody" (which has uid 99 in the base unraid OS) and group "users" (which is gid 100). I tried changing the container USER_ID from 99 to 100 and GROUP_ID from 100 to 101 but got same result. I also tried changing to 775 and also chown 100.101 but same error.
I ended up chmod'ing "/mnt/cache/appdata/clamav" to 777 which got ClamAV running but it doesn't feel like the right solution.