i have a similar installation (nextcloud +apache in docker container, plus caddy as reverse proxy).
i do have the two redir rules in my Caddyfile (outside of reverse_proxy):
redir /.well-known/carddav /remote.php/carddav 301
redir /.well-known/caldav /remote.php/caldav 301
and i get no warnings in the nextcloud admin pages
(i also have this header setting:
header Strict-Transport-Security Strict-Transport-Security "max-age=15768000; includeSubDomains; p
reload;"
), outside of the reverse_proxy
it works since a few months.