The Oracle Cloud Ubuntu images come with somewhat restrictive iptables rules by default. Docker manages the instance firewall and we have the Oracle Cloud firewall in front, so let's remove the current firewall to avoid trouble:
apt purge netfilter-persistent iptables-persistent
Oracle cloud includes a somewhat heavy monitoring daemon. We have better use for that memory since current versions of Synapse, the Matrix homeserver, can be memory hungry.
snap remove oracle-cloud-agent
apt purge snapd
apt purge open-iscsi
apt purge lxd
apt purge lxcfs