....because then containers can't network at all to non-local networks, e.g. no internet access. for bridge networks at least (which is the default).

by specifying the drop for new connections incoming from the external interface, you stop connections to listening services from external networks, but established and related connections can continue implicitly, so forwarding still works for outbound connections.

if you really want to block all internet access for containers, and stop anything else on your system that might need to use forwarding, then your suggestion is correct.