Switch to Cirrus Dockerfile as CI (flutter#13440)

.cirrus.yml

@@ -3,13 +3,15 @@ gcp_credentials: ENCRYPTED[987a78af29b91ce8489594c9ab3fec21845bbe5ba68294b8f6def
 # LINUX
 task:
   gke_container:
-   image: gcr.io/flutter-cirrus/build-engine-image:latest
-   cluster_name: build-32-cluster
-   zone: us-central1-a
-   namespace: default
-   cpu: 30 # can't use all 30-cores; system pods needs cores too
-   memory: 100Gb # similarly, can't use all 100Gb memory
+    dockerfile: "ci/docker/build/Dockerfile"
+    builder_image_name: docker-builder # gce vm image
+    cluster_name: build-32-cluster
+    zone: us-central1-a
+    namespace: default
+    cpu: 30 # can't use all 30-cores; system pods needs cores too
+    memory: 100Gb # similarly, can't use all 100Gb memory
   env:
+    CIRRUS_DOCKER_CONTEXT: "ci/docker/build"
     CIRRUS_WORKING_DIR: "/tmp/github_repo"
     ENGINE_PATH: "/tmp/clean_engine"
     DEPOT_TOOLS: "/tmp/depot_tools"

ci/docker/build/README.md

@@ -4,9 +4,16 @@ 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 internal doc go/installdocker for how to setup `docker` on gLinux.
 
-After setup,
-* edit `Dockerfile` to change how the container image is built.
-* run `./build_docker.sh` to build the container image.
-* run `./push_docker.sh` to push the image to google cloud registry. This will
-  affect our CI tests.
+Cirrus will build (and cache) a Docker image based on this `Dockerfile` for
+Linux tasks using its
+[Dockerfile as CI](https://cirrus-ci.org/guide/docker-builder-vm/) feature.
+Any change to the `Dockerfile` will cause a new task to be triggered to build
+and tag a new version of the Docker image which will be a dependency of the
+other Linux tasks. This task will instantiate a new GCP VM based on the image
+specified in the `.cirrus.yml` `builder_image_name` field.
 
+To test changes to the Linux `Dockerfile`, create a PR with the changes, and
+Cirrus will attempt to build a new image.
+
+To debug locally, you can build an image with `./build_docker.sh`, but pushing
+to the registry is no longer necessary.