"Bad Gateway" returned when running on a Raspberry Pi 4

[Zeromusta]

I've run this command to test grist-omnibus and have seen success under Debian x86:

mkdir /tmp/grist-test/
sudo docker run   -p 9999:80   -e URL=http://{hostname-here}:9999   -e TEAM=cool-beans   -e [email protected]   -e PASSWORD=topsecret   -v /tmp/grist-test:/persist   --name grist --rm   -it gristlabs/grist-omnibus

This same command won't work for me under Raspberry Pi OS Lite on a Raspberry Pi 4. It appears to be running:

pi@rpi-home:~ $ sudo docker run   -p 9999:80   -e URL=http://rpi-home:9999   -e TEAM=cool-beans   -e [email protected]   -e PASSWORD=topsecret   -v /tmp/grist-test:/persist   --name grist --rm   -it gristlabs/grist-omnibus
info [grist-omnibus] Calling traefik [
  '--providers.file.filename=/settings/traefik.yaml',
  '--entryPoints.web.address=:80'
]
2024/06/01 11:42:21 Starting up on port 17102
INFO[0000] Configuration loaded from flags.             
info [grist-omnibus] No /custom/dex.yaml
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
time="2024-06-01T11:42:21Z" level=info msg="Dex Version: v2.33.1-dirty, Go Version: go1.18.4, Go OS/ARCH: linux arm64"
time="2024-06-01T11:42:21Z" level=info msg="config using log level: debug"
time="2024-06-01T11:42:21Z" level=info msg="config issuer: http://rpi-home:9999/dex"
time="2024-06-01T11:42:21Z" level=info msg="config storage: sqlite3"
time="2024-06-01T11:42:21Z" level=info msg="config static client: Grist"
time="2024-06-01T11:42:21Z" level=info msg="config connector: google"
time="2024-06-01T11:42:21Z" level=info msg="config connector: microsoft"
time="2024-06-01T11:42:21Z" level=info msg="config connector: local passwords enabled"
time="2024-06-01T11:42:21Z" level=info msg="config skipping approval screen"
time="2024-06-01T11:42:21Z" level=info msg="config refresh tokens rotation enabled: true"
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
time="2024-06-01T11:42:21Z" level=info msg="listening (http) on 0.0.0.0:9999"
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 404
info [grist-omnibus] Checking dex... at http://rpi-home:9999/dex/.well-known/openid-configuration
debug [grist-omnibus]   got: 200
info [grist-omnibus] Happy with dex
info [grist-omnibus] Starting traefik-forward-auth
info [grist-omnibus] I think everything has started up now
info [grist-omnibus] Listening internally on 80, externally at http://rpi-home:9999

However when trying to access Grist all that is returned is Bad Gateway:

I've explored a few things, changing the port, trying IPs addresses, can't get anything to work as opposed to x86 Debain that works first time without issue. More details in the self-hosting channel in the discord https://discord.com/channels/1176642613022044301/1245827034694029323