Add backport action (elastic#599) (elastic#680)

(cherry picked from commit 3ae3304)

Co-authored-by: Gareth Ellis <[email protected]>

.backportrc.json

@@ -0,0 +1,18 @@
+{
+  "repoOwner": "elastic",
+  "repoName": "rally-tracks",
+  "targetBranchChoices": [
+    "8.15",
+    "8.14",
+    "8.13"
+  ],
+  "targetPRLabels": [
+    "backport"
+  ],
+  "branchLabelMapping": {
+    "^backport-to-(.+)$": "$1"
+  },
+  "autoMerge": true,
+  "autoMergeMethod": "squash",
+  "prDescription": "{defaultPrDescription}\n\n<!--BACKPORT {commits} BACKPORT-->"
+}

.github/workflows/backport.yml

@@ -0,0 +1,27 @@
+name: Automatic backport action
+
+on:
+  pull_request_target:
+    branches: ['master']
+    types: ["labeled", "closed"]
+
+jobs:
+  backport:
+    name: Backport PR
+    runs-on: ubuntu-latest
+    if: |
+        github.event.pull_request.merged == true
+        && !contains(github.event.pull_request.labels.*.name, 'backport')
+    steps:
+      - name: Backport Action
+        uses: sorenlouv/[email protected]
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Info log
+        if: ${{ success() }}
+        run: cat ~/.backport/backport.info.log
+
+      - name: Debug log
+        if: ${{ failure() }}
+        run: cat ~/.backport/backport.debug.log