Disable network access for the containers

Turns out setting up a network namespace and reconfiguring the firewall
is pretty expensive. Disabling this setup brings down test times from
~210s to ~65s.

test-framework/sudo-test/src/docker.rs

@@ -59,7 +59,7 @@ impl Container {
 
     pub fn new_with_hostname(image: &str, hostname: Option<&str>) -> Result<Self> {
         let mut docker_run = docker_command();
-        docker_run.args(["run", "--detach"]);
+        docker_run.args(["run", "--net=none", "--detach"]);
         if let Some(hostname) = hostname {
             docker_run.args(["--hostname", hostname]);
         }