Worth sharing one more thing to simplify complications later on. You can set the trusted_proxies value in the HomeAssistant configuration to the Docker IP Range!
This way if you ever have to change (or re-install SWAG; like I just had to do) you won't have a broken HomeAssistant instance (with no recollection of having worked all this very specific IP stuff out some time back)....
http:
use_x_forwarded_for: true
trusted_proxies:
- 172.X.0.0/12 # The IP Range for SWAG docker IP address