Add method to expose container id

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

integration/tailscale.go

@@ -20,4 +20,5 @@ type TailscaleClient interface {
 	WaitForReady() error
 	WaitForPeers(expected int) error
 	Ping(hostnameOrIP string) error
+	ID() string
 }

integration/tsic/tsic.go

@@ -176,6 +176,10 @@ func (t *TailscaleInContainer) Version() string {
 	return t.version
 }
 
+func (t *TailscaleInContainer) ID() string {
+	return t.container.Container.ID
+}
+
 func (t *TailscaleInContainer) Execute(
 	command []string,
 ) (string, string, error) {