blog, tour, go.dev, golang.org: stop referencing mirror.gcr.io URLs

As per the documentation: Google Cloud services such as Cloud Build and
Google Kubernetes Engine automatically check for cached images before
attempting to pull an image from Docker Hub.

Also: Only obtain cached images on mirror.gcr.io by configuring the
Docker daemon. A request to pull directly from mirror.gcr.io will fail
if a cached copy of the image does not exist.

Source: https://cloud.google.com/container-registry/docs/pulling-cached-images

Change-Id: I4320f97b016d928ea7431c56dd7486467f436a53
Reviewed-on: https://go-review.googlesource.com/c/website/+/340171
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Website-Publish: DO NOT USE <dmitshur@google.com>

blog/cloudbuild.yaml

@@ -3,13 +3,13 @@
 # Do not run directly.
 
 steps:
-  - name: mirror.gcr.io/library/golang
+  - name: golang
     args: ["go", "test", "./..."]
     dir: blog
   - name: gcr.io/cloud-builders/gcloud
     entrypoint: bash
     args: ["./go-app-deploy.sh", "blog/app.yaml"]
-  - name: mirror.gcr.io/library/golang
+  - name: golang
     args: [
       "go", "run", "./cmd/versionprune", "--dry_run=false",
       "--project=$PROJECT_ID", "--service=blog",

cmd/golangorg/cloudbuild.yaml

@@ -13,14 +13,14 @@ steps:
     args: [
       "archive", "--format=zip", "--output=../_goroot.zip", "HEAD",
     ]
-  - name: mirror.gcr.io/library/golang
+  - name: golang
     args: ["rm", "-rf", "_gotmp"]
-  - name: mirror.gcr.io/library/golang
+  - name: golang
     args: ["go", "test", "./..."]
   - name: gcr.io/cloud-builders/gcloud
     entrypoint: bash
     args: ["./go-app-deploy.sh", "cmd/golangorg/app.yaml"]
-  - name: mirror.gcr.io/library/golang
+  - name: golang
     args: [
       "go", "run", "./cmd/versionprune", "--dry_run=false",
       "--project=$PROJECT_ID", "--service=default",

go.dev/cloudbuild.staging.yaml

@@ -1,11 +1,11 @@
 steps:
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     entrypoint: bash
     args:
       - -c
       - go run ./cmd/events/ > ./_content/events.yaml
     dir: go.dev
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args:
       - test
       - ./...
@@ -16,9 +16,9 @@ steps:
   - name: "gcr.io/cloud-builders/gcloud"
     args: ["app", "deploy", "app.staging.learn.yaml"]
     dir: go.dev
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: ["go", "run", "./cmd/versionprune", "--project=$PROJECT_ID", "--service=staging-go-dev", "--dry_run=false"]
     dir: go.dev
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: ["go", "run", "./cmd/versionprune", "--project=$PROJECT_ID", "--service=staging-learn-go-dev", "--dry_run=false"]
     dir: go.dev

go.dev/cloudbuild.yaml

@@ -3,17 +3,17 @@
 # Do not run directly.
 
 steps:
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: ["go", "test", "./..."]
     dir: go.dev
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     entrypoint: bash
     args: ["-c", "go run ./cmd/events/ > ./_content/events.yaml"]
     dir: go.dev
   - name: "gcr.io/cloud-builders/gcloud"
     entrypoint: bash
     args: ["./go-app-deploy.sh", "--project=go-discovery", "go.dev/app.yaml"]
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: [
       "go", "run", "./cmd/versionprune", "--dry_run=false",
       "--project=go-discovery", "--service=go-dev",

tour/cloudbuild.yaml

@@ -3,13 +3,13 @@
 # Do not run directly.
 
 steps:
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: ["go", "test", "./..."]
     dir: tour
   - name: gcr.io/cloud-builders/gcloud
     entrypoint: bash
     args: ["./go-app-deploy.sh", "tour/app.yaml"]
-  - name: 'mirror.gcr.io/library/golang'
+  - name: 'golang'
     args: [
       "go", "run", "./cmd/versionprune", "--dry_run=false",
       "--project=$PROJECT_ID", "--service=tour",