infra: actions: fix docker changes detector so it doesnt always build on force pushes

.gitea/workflows/gentoo-utils.yml

@@ -28,8 +28,8 @@ jobs:
           image_tag="$(echo "$image_tag" | sed -E 's/[^a-zA-Z0-9]/-/g')"
 
           # rebase breaks gitea.event.before, so check to make sure the hash provided exists
-          if ! git merge-base --is-ancestor $comparison_hash origin/$branch_name >/dev/null 2>&1; then
+          if ! git merge-base --is-ancestor $comparison_hash $branch_name >/dev/null 2>&1; then
-            comparison_hash=$(git merge-base ${{ gitea.event.repository.default_branch }} $branch_name)
+            comparison_hash=$(git merge-base origin/$default_branch_name $branch_name)
           fi
             
           if ! git diff $comparison_hash ${{ gitea.sha }} --no-patch --exit-code .docker; then