How enable html inline in Horde Groupware Webmail Edition

You can edit the files on the host, see my Active Sync example:

Inline HTML configuration works in the same way, in my case the setting was in line 50 in
/var/lib/docker/overlay/c08b6b5ce52d7fc78690ea5bb00d913b821bcfae8c7a42e35c1835883a39cd55/upper/etc/horde/imp/mime_drivers.php