Ok, I've solved this for my purposes.
Seems like when I try curl -v https://bind.0data.app/CloudronVersions.json from my Cloudron, it fails.
* Trying •••.••.••.••:443...
* connect to •••.••.••.•• port 443 failed: Connection timed out
* Failed to connect to bind.0data.app port 443 after 129437 ms: Connection timed out
* Closing connection 0
curl: (28) Failed to connect to bind.0data.app port 443 after 129437 ms: Connection timed out
Which tells me there's some issue with loading URLs from the same server.
Interestingly, if I find the internal IP via
docker inspect <process> | grep '"IPAddress"'
and port via
docker inspect <process> | grep -A5 '"Ports"'
I can call successfully load the file with
curl -v -H "Host: bind.0data.app" http://<ip>:<port>/CloudronVersions.json
But I have decided it's easier for me to simply host a kind of URL masking thing on another server. Anyway thanks for the help and attention from everyone who chimed in ️