Putting a QEMU guest onto a network created by docker compose - docker

Question

I am trying to emulate a hardware configuration between two Intel NUC servers and a Raspberry Pi server using docker containers and docker compose. Since the latter is ARM, and the test host is x86, I decided to run the RPi image within QEMU encapsulated in one of the docker containers. The docker compose file is pretty simple:
services:
rpi:
build: ./rpi
networks:
iot:
ipv4_address: 192.168.1.3
ether:
ipv4_address: 192.168.2.3
nuc1:
build: ./nuc
networks:
- "iot"
nuc2:
build: ./nuc
networks:
- "ether"
networks:
iot:
driver: bridge
ipam:
driver: default
config:
- subnet: 192.168.1.0/24
gateway: 192.168.1.1
ether:
driver: bridge
ipam:
driver: default
config:
- subnet: 192.168.2.0/24
gateway: 192.168.2.1
You'll notice that there are two separate networks, I have two vlans I am trying to emulate and the rpi server is apart of both, hence the docker container is a part of both. I have QEMU running fine on the rpi container, and the docker containers themselves are behaving on the network as intended.
The problem I am having is trying to set up the networking so that the QEMU image appears to be just another node on the network at the addresses 192.168.1.3/192.168.2.3. My requirements are that:
The QEMU guest knows that its own IP on each network is 192.168.1.3 and 192.168.2.3 respectively.
The other NUC docker containers can reach the QEMU image at those IP address (i.e. I don't want to give the docker container running the QEMU image its own IP address, have the other containers hit that IP address, then NAT the address to the QEMU address).
I tried the steps listed on this gist with no luck. Additionally, I tried creating a TAP with an address of 10.0.0.1 on the QEMU docker container, a bound the QEMU guest to at TAP, then creating an IP tables rule to NAT traffic received to the TAP, however, the issue is that then the destination address is 10.0.0.1 and the QEMU guest thinks its own address is 192.168.1.3, so it won't receive the packet.
As you can see, I am a bit stuck conceptually, and need some help with a direction to take this.
Right now, this is the network configuration that I set up to handle traffic on the QEMU container (excuse the lack of consistent ip usage, iproute2 is not installed on the image I am using and I can't seem to get the containers to hit the internet):
brctl addbr br0
ip addr flush dev eth0
ip addr flush dev eth1
brctl addif br0 eth0
brctl addif br0 eth1
tunctl -t tap0 -u $(whoami)
brctl addif br0 tap0
ifconfig br0 up
ifconfig tap0 up
ip addr add 192.168.1.3 dev br0
ip addr add 192.168.2.3 dev br0
ip route add 192.168.1.1/24 dev br0
ip route add 192.168.2.1/24 dev br0
ip addr add 10.0.0.1 dev tap0
Then I've done the following forwarding rules:
iptables -t nat -A PREROUTING -i br0 -d 192.168.1.1 -j DNAT --to 10.0.0.1
iptables -t nat -A POSTROUTING -o br0 -s 10.0.0.1 -j SNAT --to 192.168.1.1
iptables -t nat -A PREROUTING -i br0 -d 192.168.2.1 -j DNAT --to 10.0.0.1
iptables -t nat -A POSTROUTING -o br0 -s 10.0.0.1 -j SNAT --to 192.168.2.1