docker (prosody): fixed prosodyctl call in container_server: it was not using absolute path, so it was calling /usr/local/bin/prosodyctl instead of /usr/bin/prosodyctl, resulting in a loop

#!/bin/sh
# account domain is set, then sat is launched with D-Bus activated
chmod a+w /etc/hosts
su -c "set_account_domain && dbus-launch /usr/bin/sat $@" sat