Load the web form in a browser. Look at the browser's inspector. Any errors there?
Manually run the following commands to check what's happening inside the Enketo container.
# connect to enketo container
cd ~/central
docker exec -it central-enketo-1 bash
# install curl
apt update
apt install curl --assume-yes
# run curl. make sure to use your own domain. 1 means everything is working.
curl --silent --insecure https://your.central.domain | grep --count getodk.org
# clean up
apt remove curl --assume-yes
apt autoremove --assume-yes