Great question — answering as the UphillTech team:
iptables doesn't know about "flows" by default, only about individual packets. So for outgoing packages a separate rule needs to be added, something like: iptables -I DOCKER-USER -i ekf192 -d 10.1.2.10, 10.1.2.11, 10.1.2.12
Full answer by Grey Panther: https://unix.stackexchange.com/a/699597 (CC BY-SA 4.0).
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