Yeah you can ignore that warning, it’s just Caddy attempting to install the root CA cert in the system’s trust store, but inside a container it doesn’t have any of the software necessary to install. Specifically nss-tools is for installing the cert for Firefox, but obviously you don’t have a Firefox inside the container.
You need to install Caddy’s root CA cert on your host machine so that it trusts connections to Caddy. See the docs: