ci: fix logic for detecting changes to .docker in build-oci-image

.gitea/workflows/gentoo-utils.yml

@@ -24,13 +24,39 @@ jobs:
 
       - name: Check for changes before building
         id: image-changes
+        # build image only if 1. changes are detected or 2. an image for the working branch doesnt exist
         run: |
-          echo "branch_name=${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}" >> $GITEA_OUTPUT
-          if ! git diff ${{ gitea.event.before }} ${{ gitea.sha }} --no-patch --exit-code .docker; then
-            echo changes_detected=true >> $GITEA_OUTPUT
-          else
-            echo changes_detected=false >> $GITEA_OUTPUT
+          branch_name="${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" != "$default_branch_name" ]]; then
+            image_tag=$branch_name
           fi
+          # slugify
+          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 $branch_name >/dev/null 2>&1; then
+            comparison_hash=$(git merge-base origin/$default_branch_name $branch_name)
+          fi
+            
+          if ! git diff $comparison_hash ${{ gitea.sha }} --no-patch --exit-code .docker; then
+            build_image=true
+          else
+            if ! docker manifest inspect ${{ vars.REGISTRY_URL }}/${{ gitea.repository }}:${image_tag} >/dev/null 2>&1; then
+              build_image=true
+            else
+              build_image=false
+            fi
+          fi
+
+          echo "default_branch_name=$default_branch_name" >> $GITEA_OUTPUT
+          echo "branch_name=$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
         if: steps.image-changes.outputs.changes_detected == 'true'