This makes it easier to use the docker container smoke test that
GitHub actions runs. There is also `make smoke-docker-race` that runs the
smoke test with `-race` enabled.
Test that basic inbound / outbound firewall rules work during the smoke
test. This change sets an inbound firewall rule on host3, and a new
host4 with outbound firewall rules. It also tests that conntrack allows
packets once the connection has been established.