Update docker image tag and README (#5995)

.cirrus.yml

@@ -32,7 +32,7 @@ task:
 
 format_and_dart_test_task:
   container:
-    image: gcr.io/flutter-cirrus/build-engine-image:0.2
+    image: gcr.io/flutter-cirrus/build-engine-image:latest
 
   env:
     CIRRUS_WORKING_DIR: "/tmp/github_repo"

ci/docker/build/README.md

@@ -2,8 +2,7 @@ This directory includes scripts to build the docker container image used for
 building flutter/engine in our CI system (currently [Cirrus](cirrus-ci.org)).
 
 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.
+refer to internal doc go/installdocker for how to setup `docker` on gLinux.
 
 After setup,
 * edit `Dockerfile` to change how the container image is built.