Replace every occurrence of eth0 and/or wlan0 with your interface names only if they differ in the following:

#!/bin/bash
iptables -I DOCKER-USER -i src_if -o dst_if -j ACCEPT
iptables -t nat -C POSTROUTING -o eth0 -j MASQUERADE || iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -C FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT || iptables -A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -C FORWARD -i wlan0 -o eth0 -j ACCEPT || iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT
iptables-save

Got it. Sorry if this is a stupid question,. It looks like I have a docker0, eth0, wlan0, and a wlan1 if I am in the OS (but that changes to wlxdxyz inside RaspAP docker container). Is it appropriate to replace eth0 with wlxdxyz? Thanks for taking the time with me here, I appreciate it!

These rules apply to the host's network interfaces, not Docker (as mentioned in the docs).

I'm sorry to keep having to come back. I've tried a lot of different things and just keep failing, so thought I would describe here a bit more detail.

Goal: I am trying to do a wireless repeater from wlxd123 > wlan0. It works on standalone OS. I then tried the Docker install.

"ip a" in either the host terminal or the docker pseudo terminal tells me I have (1) lo (2) end0 (3) wlan0 (4) wlxd123 (5) docker0.

I first tried replacing eth0 with end0 and wlan0 with wlxd123. I rebooted for good measure and that didn't do anything. I then replaced end0 with wlan1 because I did see that at some point and that didn't do anything, except that the host RPi OS keeps asking to authenticate my WiFi network and won't connect. I then replaced it back to the original firewall-rules.sh files and executed that and rebooted and it still won't authenticate my host WiFi, which has happened on and off before. I then tried replacing eth0 with docker0 and wlan0 with wlxd123 and that failed yet again.

Also to note, "sudo iptables -S" does not show the rules I created after restarting, but maybe I'm not looking correctly. Now, no matter what I try, my Wifi also never authenticates properly to any network.

Any thoughts about what I'm doing wrong?

If you don't need an isolated Docker container, I'd strongly suggest using a pre-built image instead.