add new docker_attach.sh script and update README (#61363)

dev/ci/docker_linux/README.md

@@ -5,8 +5,11 @@ In order to run the scripts, you have to setup `docker` and `gcloud`. Please
 refer to the [internal flutter team doc](go/flutter-team) for how to setup in a
 Google internal environment.
 
-After setup,
-* edit `Dockerfile` to change how the container image is built.
-* run `./docker_build.sh` to build the container image.
-* run `./docker_push.sh` to push the image to google cloud registry. This will
-  affect our CI tests.
+To debug the image locally:
+* (Optional) edit the `Dockerfile` to change how the container image is built.
+* Run `./docker_build.sh` to build the container image (`sudo` permission is
+  required)
+* Run `./docker_attach.sh` to start a container from the image and attach to its
+  internal bash shell. From here, you can invoke shell commands from the
+  `.cirrus.yml` (you will have to manually run any `setup` steps; e.g. the
+  container will not have the Flutter repo cloned yet).

dev/ci/docker_linux/docker_attach.sh

+#!/usr/bin/env bash
+# Copyright 2014 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+TAG="${CIRRUS_TAG:-latest}"
+
+# Starts an interactive docker container with a bash shell running in it, and
+# attaches the user's shell to it.
+sudo docker run --interactive --tty \
+  "gcr.io/flutter-cirrus/build-flutter-image:$TAG" \
+  /bin/bash