ci: make it work for forks too

.github/workflows/continuous-integration.yml

@@ -161,12 +161,29 @@ jobs:
           platform: linux/arm/v7
           alternate_repository: snapshot
 
-  merge_clean_snapshot_tags:
+  merge_clean_snapshot_guard:
     needs:
       - snapshot_amd64
       - snapshot_386
       - snapshot_arm64
       - snapshot_arm_v7
+    name: Secrets access check
+    runs-on: ubuntu-latest
+    outputs:
+      continue: ${{ steps.check.outputs.continue }}
+    steps:
+      - name: Check
+        id: check
+        run: |
+          if [ -n "${{ secrets.DOCKERHUB_USERNAME }}" ]; then
+            echo "continue=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "continue=false" >> "$GITHUB_OUTPUT"
+          fi
+
+  merge_clean_snapshot_tags:
+    if: needs.merge_clean_snapshot_guard.outputs.continue == 'true'
+    needs: merge_clean_snapshot_guard
     name: Merge and clean snapshot tags
     runs-on: ubuntu-latest
     steps: