Add host.containers.internal entry into container's etc/hosts

This change adds the entry `host.containers.internal` to the `/etc/hosts`
file within a new containers filesystem. The ip address is determined by
the containers networking configuration and points to the gateway address
for the containers networking namespace.

Closes #5651

Signed-off-by: Baron Lenardson <lenardson.baron@gmail.com>

libpod/container.go

@@ -126,6 +126,8 @@ type Container struct {
 
 	// This is true if a container is restored from a checkpoint.
 	restoreFromCheckpoint bool
+
+	slirp4netnsSubnet *net.IPNet
 }
 
 // ContainerState contains the current state of the container