Add automation for updating builder lifecycle version

.github/workflows/update-lifecycle.yml

@@ -0,0 +1,71 @@
+name: Update lifecycle
+
+on:
+  schedule:
+    # Every Monday at 8am UTC.
+    - cron: '0 8 * * MON'
+  workflow_dispatch:
+
+jobs:
+  update-lifecycle:
+    name: Update lifecycle
+    runs-on: pub-hk-ubuntu-22.04-small
+    steps:
+      - name: Get token for GH application (Linguist)
+        uses: heroku/use-app-token-action@main
+        id: generate-token
+        with:
+          app_id: ${{ vars.LINGUIST_GH_APP_ID }}
+          private_key: ${{ secrets.LINGUIST_GH_PRIVATE_KEY }}
+
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          token: ${{ steps.generate-token.outputs.app_token }}
+
+      - name: Record existing lifecycle version
+        id: existing-version
+        # Having to use grep instead of yq due to:
+        # https://github.com/mikefarah/yq/issues/1758
+        run: echo "version=$(grep --perl-regexp --null-data --only-matching '\[lifecycle\]\nversion = "\K([^"]+)' builder-22/builder.toml)" >> "${GITHUB_OUTPUT}"
+
+      - name: Determine latest lifecycle version
+        id: latest-version
+        run: echo "version=$(gh release view --repo buildpacks/lifecycle --json tagName --jq '.tagName | sub("v"; "")')" >> "${GITHUB_OUTPUT}"
+
+      - name: Update builder manifests with latest lifecycle version
+        # This only updates manifests that were on the same version as builder-22, to ensure
+        # that any legacy builder images pinned to older lifecycle versions are not updated too.
+        run: sed --in-place --expression 's/^version = "${{ steps.existing-version.outputs.version }}"$/version = "${{ steps.latest-version.outputs.version }}"/' */builder.toml
+
+      # This step will skip creating a PR if there are no changes to commit.
+      - name: Create pull request
+        id: pr
+        uses: peter-evans/[email protected]
+        with:
+          token: ${{ steps.generate-token.outputs.app_token }}
+          title: Update lifecycle from v${{ steps.existing-version.outputs.version }} to v${{ steps.latest-version.outputs.version }}
+          body: |
+            Release notes:
+            https://github.com/buildpacks/lifecycle/releases/tag/v${{ steps.latest-version.outputs.version }}
+
+            Full changelog:
+            https://github.com/buildpacks/lifecycle/compare/v${{ steps.existing-version.outputs.version }}...v${{ steps.latest-version.outputs.version }}
+          commit-message: |
+            Update lifecycle from v${{ steps.existing-version.outputs.version }} to v${{ steps.latest-version.outputs.version }}
+
+            Release notes:
+            https://github.com/buildpacks/lifecycle/releases/tag/v${{ steps.latest-version.outputs.version }}
+
+            Full changelog:
+            https://github.com/buildpacks/lifecycle/compare/v${{ steps.existing-version.outputs.version }}...v${{ steps.latest-version.outputs.version }}
+          branch: update-lifecycle
+          delete-branch: true
+          committer: ${{ vars.LINGUIST_GH_APP_USERNAME }} <${{ vars.LINGUIST_GH_APP_EMAIL }}>
+          author: ${{ vars.LINGUIST_GH_APP_USERNAME }} <${{ vars.LINGUIST_GH_APP_EMAIL }}>
+
+      - name: Enable PR auto-merge
+        if: steps.pr.outputs.pull-request-operation == 'created'
+        run: gh pr merge --auto --squash "${{ steps.pr.outputs.pull-request-number }}"
+        env:
+          GH_TOKEN: ${{ steps.generate-token.outputs.app_token }}