[Added]add-gcp-cloudbuild-instruction (#135)

Hi

I write some instrction about how to use gcr-cleaner in GCP cloudbuild.
I think it may be necessary. Because I spend some time to try how to use
and README.md doesn't have instrction about it

README.md

@@ -7,6 +7,7 @@ deployment option for a detailed guide:
 
 - [Scheduled GitHub Action workflow](docs/deploy-github-actions.md)
 - [Deployed to Cloud Run](docs/deploy-cloud-run.md)
+- [Deploy GCR Cleaner to Cloudbuild](docs/deploy-gcp-cloudbuild.md)
 
 For one-off tasks, you can also run GCR Cleaner locally:
 

docs/deploy-gcp-cloudbuild.md

@@ -0,0 +1,39 @@
+# Deploy GCR Cleaner to Cloudbuild
+
+This document describes how to use GCR Cleaner in GCP [Cloud Build][cloud-build] with [Artifact Registry][artifact-registry]
+
+1. Grant a role `roles/artifactregistry.repoAdmin` to the [Cloud Build service account][cloud-build-service-account] 
+   because it need `artifactregistry.repositories.deleteArtifacts` permission.
+
+1. Export your project ID as an environment variable.
+
+   ```sh
+   export PROJECT_ID="my-project"
+   ```
+
+1. Create a YAML file named cloudbuild.yaml which will always keep three images:
+
+    ```yaml
+    steps:
+      - name: asia-docker.pkg.dev/gcr-cleaner/gcr-cleaner/gcr-cleaner-cli:latest
+        args:
+          - -repo
+          - "asia-docker.pkg.dev/my-project/my-repo/my-image"
+          - -keep
+          - "3"
+          - -tag-filter-any
+          - ".*"
+    ```    
+
+1. Manual trigger Cloud Build using [gcloud CLI][cloud-cli] to check it:
+
+   ```sh
+   gcloud builds submit \
+     --project "${PROJECT_ID}" \
+     --config cloudbuild.yaml .
+   ```
+
+[cloud-build]: https://cloud.google.com/build
+[artifact-registry]: https://cloud.google.com/artifact-registry
+[cloud-cli]: https://cloud.google.com/cli
+[cloud-build-service-account]: https://cloud.google.com/build/docs/cloud-build-service-account