ci(github): add new workflow for backporting (#400)

.github/workflows/backport.yml

@@ -0,0 +1,49 @@
+name: Backport labeled merged pull requests
+on:
+  pull_request:
+    types: [closed]
+  issue_comment:
+    types: [created]
+jobs:
+  build:
+    name: Create backport PRs
+    runs-on: ubuntu-latest
+    # Only run when pull request is merged
+    # or when a comment containing `/backport` is created
+    if: >
+      (
+        github.event_name == 'pull_request' &&
+        github.event.pull_request.merged
+      ) || (
+        github.event_name == 'issue_comment' &&
+        github.event.issue.pull_request &&
+        contains(github.event.comment.body, '/backport')
+      )
+    steps:
+      - uses: actions/checkout@v2
+      - name: Create backport PRs
+        uses: zeebe-io/backport-action@v0.0.7
+        with:
+          # Required
+          # Token to authenticate requests to GitHub
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+          # Required
+          # Working directory for the backport action
+          github_workspace: ${{ github.workspace }}
+
+          # Optional
+          # Regex pattern to match github labels
+          # Must contain a capture group for target branchname
+          # label_pattern: ^backport ([^ ]+)$
+
+          # Optional
+          # Template used as description in the pull requests created by this action.
+          # Placeholders can be used to define variable values.
+          # These are indicated by a dollar sign and curly braces (`${placeholder}`).
+          # Please refer to this action's README for all available placeholders.
+          pull_description: |-
+            # Description
+            Backport of #${pull_number} to `${target_branch}`.
+            
+            relates to ${issue_refs}