add cron pipeline to conditionally trigger main pipeline

.drone.yml

@@ -1,11 +1,58 @@
+---
+kind: pipeline
+type: docker
+name: check
+trigger:
+  event:
+    - cron
+
+steps:
+  - name: check
+    image: docker:latest
+    volumes:
+      - name: docker-socket
+        path: /var/run/docker.sock
+    environment:
+      DRONE_API_TOKEN:
+        from_secret: drone_api_token
+      REGISTRY_PWD:
+        from_secret: registry_pwd
+    commands:
+      - apk add git curl
+      # collect info from upstream repository
+      - git clone https://github.com/drone/drone.git
+      - cd drone
+      - LATEST_VERSION=$(git tag --sort v:refname | grep '^v[0-9]\{1,\}\.[0-9]\{1,\}\.[0-9]\{1,\}$' | tail -n 1)
+      - git checkout $LATEST_VERSION
+      - COMMIT_HASH=$(git log -n 1 --pretty=%H)
+      # collect info from current image
+      - echo $REGISTRY_PWD | docker login -u drone --password-stdin registry.jfmonty2.com
+      - docker pull registry.jfmonty2.com/drone:latest
+      - IMAGE_COMMIT_HASH=$(docker image inspect -f '{{ .Config.Labels.drone_commit_hash }}' registry.jfmonty2.com/drone:latest)
+      # compare
+      - |
+        if [[ $COMMIT_HASH != $IMAGE_COMMIT_HASH ]]; then
+          echo 'Updates available, triggering build pipeline.'
+          curl -X POST -H "Authorization: Bearer $DRONE_API_TOKEN" \
+          https://drone.jfmonty2.com/api/repos/jfmonty2/drone/builds
+        else
+          echo 'Image is already up to date.'
+        fi
+
+volumes:
+  - name: docker-socket
+    host:
+      path: /var/run/docker.sock
+
+
 ---
 kind: pipeline
 type: docker
 name: main
 
 trigger:
-  branch:
-    - master
+  event:
+    - custom
 
 steps:
   - name: prepare
@@ -34,7 +81,11 @@ steps:
       GOOS: linux
     commands:
       - cd drone
-      - go build -ldflags "-extldflags \"-static\"" -tags nolimit -o release/linux/amd64/drone-server github.com/drone/drone/cmd/drone-server
+      - >
+        go build -ldflags "-extldflags \"-static\""
+        -tags nolimit
+        -o release/linux/amd64/drone-server
+        github.com/drone/drone/cmd/drone-server
 
   - name: docker-build
     image: docker:latest
@@ -44,7 +95,13 @@ steps:
     commands:
       - cd drone
       - rm .dockerignore
-      - docker build -t registry.jfmonty2.com/drone-server:latest -f docker/Dockerfile.server.linux.amd64 .
+      - COMMIT_HASH=$(git log -n 1 --pretty=%H)
+      - >
+        docker build 
+        -f docker/Dockerfile.server.linux.amd64
+        -t registry.jfmonty2.com/drone-server:latest 
+        --label drone_commit_hash=$COMMIT_HASH
+        .
 
   - name: deploy
     image: docker:latest