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

.gitea/workflows/gentoo-utils.yml

@@ -17,19 +17,22 @@ jobs:
         # build image only if 1. changes are detected or 2. an image for the working branch doesnt exist
         run: |
           branch_name="${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}"
+          default_branch_name="${{ gitea.event.repository.default_branch }}"
+          image_tag=latest
+          comparison_hash="${{ gitea.event.before }}"
 
-          if [[ "$branch_name" == "${{ gitea.event.repository.default_branch }}" ]]; then
-            image_tag=latest
-          else
+          if [[ "$branch_name" != "${{ gitea.event.repository.default_branch }}" ]]; then
             image_tag=$branch_name
           fi
-
-          echo "branch_name=$branch_name" >> $GITEA_OUTPUT
           # slugify
           image_tag="$(echo "$image_tag" | sed -E 's/[^a-zA-Z0-9]/-/g')"
-          echo "image_tag=$image_tag" >> $GITEA_OUTPUT
 
-          if ! git diff ${{ gitea.event.before }} ${{ gitea.sha }} --no-patch --exit-code .docker; then
+          # rebase breaks gitea.event.before, so check to make sure the hash provided exists
+          if ! git merge-base --is-ancestor $comparison_hash $branch_name; then
+            comparison_hash=$(git merge-base ${{ gitea.event.repository.default_branch }} $branch_name)
+          fi
+            
+          if ! git diff $comparison_hash ${{ gitea.sha }} --no-patch --exit-code .docker; then
             build_image=true
           else
             if ! docker manifest inspect ${{ vars.REGISTRY_URL }}/${{ gitea.repository }}:${image_tag} >/dev/null 2>&1; then
@@ -39,6 +42,10 @@ jobs:
             fi
           fi
 
+          echo "default_branch_name=$default_branch_name"
+          echo "branch_name=$branch_name" >> $GITEA_OUTPUT
+          echo "image_tag=$image_tag" >> $GITEA_OUTPUT
+          echo "comparison_hash=$comparison_hash" >> $GITEA_OUTPUT
           echo "build_image=$build_image" >> $GITEA_OUTPUT
           cat $GITEA_OUTPUT
       - name: Set up Docker buildx