Extract dockery bits from Cypress' synapsedocker plugin (#9103)

* Extract dockery bits from Cypress' synapsedocker plugin

* Update cypress/plugins/docker/index.ts

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

cypress/plugins/utils/port.ts

@@ -0,0 +1,27 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import * as net from "net";
+
+export async function getFreePort(): Promise<number> {
+    return new Promise<number>(resolve => {
+        const srv = net.createServer();
+        srv.listen(0, () => {
+            const port = (<net.AddressInfo>srv.address()).port;
+            srv.close(() => resolve(port));
+        });
+    });
+}