I wasn't so much interested in "how can I
download that myself" as in "how can I
make that myself?"
However, your link led me to
this Dockerfile, which shows one way: wrap QEMU to boot RouterOS VDI images inside the container. I was hoping for something more clever, like unpacking native x86 RouterOS filesystems into the container's rootfs, then arranging for that to boot at container start.
If it's using port 115 he can simply map that if I understand correctly.
We aren't talking about TCP/IP port 115, nor about UDP/IP port 115, but about IP protocol number 115.
Line 3, second field. I believe the core of the OP's complaint stems from the fact that there is no parallel to "docker create --port" for forwarding entire IP protocol numbers into the container, such as for consumption by a low-level networking OS like RouterOS.
Thus my suggestion to expose the naked container to the host's network stack.