fix: refer to the correct branch

Author: OlaBekkevold

.github/workflows/build-push.yml

@@ -14,6 +14,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v5
+        with:
+          ref: ${{ github.event.workflow_run.head_sha }}
 
       - name: Log in to container registry
         uses: docker/login-action@v3
@@ -25,8 +27,8 @@ jobs:
       - name: Determine image tag
         id: tag
         run: |
-          REF="${{ github.ref }}"
-          if [ "$REF" = "refs/heads/main" ]; then
+          BRANCH="${{ github.event.workflow_run.head_branch }}"
+          if [ "$BRANCH" = "main" ]; then
             echo "env_tag=latest" >> $GITHUB_OUTPUT
           else
             echo "env_tag=test" >> $GITHUB_OUTPUT