Since syslog is a UDP based protocol, the MTU size might also play a role here.
My docker image has:
# ip link list lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
Where my host system (Mac) has:
MacBook-Pro:lcomplex-100 thijs$ ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
So seems with Docker the image get’s it’s own MTU, just not sure whether the loopback for the container also goes through the host, since that would still result in fragmentation.
What is the size of your message? and what are the MTU sizes you see on your system (both in the container as well as the host)?