sanitize branch name

Author: ikraemer-dd

.github/workflows/delete-pr-image.yml

@@ -19,6 +19,10 @@ jobs:
         run: |
           BRANCH="${{ github.event.pull_request.head.ref }}"
           TAG="${BRANCH//\//_}"
+          if ! [[ $TAG =~ ^[A-Za-z0-9._-]+$ ]]; then
+            echo "Exiting - Manages branches only if they contain letters, numbers and the following special characters: . - _"
+            exit 1
+          fi
           echo "tag=$TAG" >> "$GITHUB_OUTPUT"
 
       - name: Delete image by tag from GHCR