ci: add Docker layer caching to speed up builds

Author: robertodauria

cloudbuild.yaml

@@ -5,18 +5,30 @@ substitutions:
   _TOKEN_EXCHANGE_URL: https://auth.mlab-sandbox.measurementlab.net/v0/token/integration
 
 steps:
-  # 1. Build the container image
+  # 1. Pull cached image (if exists) for layer caching
+  - name: "gcr.io/cloud-builders/docker"
+    id: PullCache
+    entrypoint: bash
+    args: ["-c", "docker pull ${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:latest || exit 0"]
+
+  # 2. Build the container image with cache
   - name: "gcr.io/cloud-builders/docker"
     id: Build
-    args: ["build", "-t", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:$BUILD_ID", "-t", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:latest", "."]
+    args: [
+      "build",
+      "-t", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:$BUILD_ID",
+      "-t", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:latest",
+      "--cache-from", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:latest",
+      "."
+    ]
 
-  # 2. Push to Artifact Registry
+  # 3. Push to Artifact Registry
   - name: "gcr.io/cloud-builders/docker"
     id: Push
     args: ["push", "${_REGION}-docker.pkg.dev/${PROJECT_ID}/m-lab/${_SERVICE_NAME}:${BUILD_ID}"]
     waitFor: ["Build"]
 
-  # 3. Deploy to Cloud Run
+  # 4. Deploy to Cloud Run
   - name: "gcr.io/google.com/cloudsdktool/cloud-sdk"
     id: Deploy
     entrypoint: gcloud