Move some helper functions into dockertestutil package

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

integration/dockertestutil/network.go

@@ -0,0 +1,21 @@
+package dockertestutil
+
+import "github.com/ory/dockertest/v3"
+
+func GetFirstOrCreateNetwork(pool *dockertest.Pool, name string) (*dockertest.Network, error) {
+	networks, err := pool.NetworksByName(name)
+	if err != nil || len(networks) == 0 {
+		if _, err := pool.CreateNetwork(name); err == nil {
+			// Create does not give us an updated version of the resource, so we need to
+			// get it again.
+			networks, err := pool.NetworksByName(name)
+			if err != nil {
+				return nil, err
+			}
+
+			return &networks[0], nil
+		}
+	}
+
+	return &networks[0], nil
+}