build: create dispatcher docker image that prefetches and prebuilds (#2151)

Towards #2142

After publishing, we will need to update the cloudbuild.yaml files to
use the dispatcher image instead of the golang base image.

Author: chingor13

cloudbuild-exitgate-dispatcher.yaml

@@ -0,0 +1,33 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# This Cloud Build configuration is used by a Louhi flow for the Artifact
+# Registry (AR) Exit Gate process (go/cloud-sdk-ar-exit-gate-onboarding).
+#
+# This build step creates the librarian container image and publishes it to the
+# 'images-dev' repository, which serves as the entry point for the AR Exit Gate.
+# After passing the gate's security checks, the image is promoted and
+# published to the 'images-prod' repository.
+steps:
+  - id: build-dispatcher
+    name: 'gcr.io/cloud-builders/docker'
+    waitFor: ['-']
+    args:
+      - 'build'
+      - '-t'
+      - 'us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-dispatcher'
+      - '.'
+      - '-f'
+      - 'infra/dispatcher/Dockerfile'
+images:
+  - 'us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-dispatcher'

cloudbuild-test.yaml

@@ -22,7 +22,8 @@
 # Local test:
 #   gcloud --project=<my-project> builds submit . --config=cloudbuild-test.yaml
 steps:
-  - name: 'gcr.io/cloud-builders/docker'
+  - id: 'build'
+    name: 'gcr.io/cloud-builders/docker'
     waitFor: ['-']
     args:
       - 'build'
@@ -31,9 +32,20 @@ steps:
       - '.'
   - id: structure-test
     name: gcr.io/gcp-runtimes/structure_test
+    waitFor: ['build']
     args:
       - '-i'
       - 'us-central1-docker.pkg.dev/$PROJECT_ID/librarian'
       - '--config'
       - '/workspace/container-structure-test.yaml'
       - '-v'
+  - id: 'build-dispatcher'
+    name: 'gcr.io/cloud-builders/docker'
+    waitFor: ['-']
+    args:
+      - 'build'
+      - '-t'
+      - 'us-central1-docker.pkg.dev/$PROJECT_ID/librarian-dispatcher'
+      - '.'
+      - '-f'
+      - 'infra/dispatcher/Dockerfile'

infra/dispatcher/Dockerfile

@@ -0,0 +1,21 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+FROM golang:1.25.0
+
+WORKDIR /app
+
+ADD . /app
+RUN go build ./...
+
+ENTRYPOINT [ "go" ]