Add new action Check PRs if on staging

.github/workflows/check_prs_if_on_staging.yml

@@ -0,0 +1,33 @@
+name: Check PRs if on staging
+on:
+    pull_request_target:
+      types: [opened, edited]
+permissions: {}
+
+jobs:
+  is_not_staging:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.base.ref != 'staging' #check if the target branch is not staging
+    steps:
+      - name: Send message
+        uses: thollander/actions-comment-pull-request@main
+        with:
+          GITHUB_TOKEN: ${{ secrets.CHECKIFPRISSTAGING_ACTION_PAT }}
+          message: |
+                   Thanks for contributing!
+
+                   I noticed that you didn't select `staging` as your base branch. Please change the base branch to `staging`.
+                   See the attached picture on how to change the base branch to `staging`:
+
+                   ![check_prs_if_on_staging.png](https://raw.githubusercontent.com/mailcow/mailcow-dockerized/master/.github/workflows/assets/check_prs_if_on_staging.png)
+
+      - name: Fail #we want to see failed checks in the PR
+        if: ${{ success() }} #set exit code to 1 even if commenting somehow failed
+        run: exit 1
+
+  is_staging:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.base.ref == 'staging' #check if the target branch is staging
+    steps:
+      - name: Success
+        run: exit 0