I’ve managed to get tcp_fastopen=3 inside my docker container now by altering my run command to include a sysctl argument:
docker run -d --restart unless-stopped --stop-timeout 300 \
--user 1000:1000 \
--sysctl net.ipv4.tcp_fastopen=3 \
-p 28969:28967/tcp \
...
docker exec -it storagenode cat /proc/sys/net/ipv4/tcp_fastopen
3
No idea if tcp_fastopen is actually being used though as netstat -s on the host doesn’t show anything and netstat doesn’t exist inside the container.