ci: use registry url variable, not hardcoded url

ci: build-oci-image: change changes_detected var to build image

ci: build-oci-image: output an image tag for other jobs to use

.gitea/workflows/gentoo-utils.yml

@@ -59,24 +59,26 @@ jobs:
           echo "build_image=$build_image" >> $GITEA_OUTPUT
           cat $GITEA_OUTPUT
       - name: Set up Docker buildx
-        if: steps.image-changes.outputs.changes_detected == 'true'
+        if: steps.image-changes.outputs.build_image == 'true'
         uses: docker/setup-buildx-action@v3
 
       - name: Log in to Github Container Registry
-        if: steps.image-changes.outputs.changes_detected == 'true'
+        if: steps.image-changes.outputs.build_image == 'true'
         uses: docker/login-action@v3
         with:
-          registry: git.epenguin.net
+          registry: ${{ vars.REGISTRY_URL }}
           username: ${{ vars.CI_BOT_USERNAME }}
           password: ${{ secrets.CI_BOT_TOKEN }}
 
       - name: Build and push
-        if: steps.image-changes.outputs.changes_detected == 'true'
+        if: steps.image-changes.outputs.build_image == 'true'
         uses: docker/build-push-action@v6
         with:
           push: true
-          tags: git.epenguin.net/${{ gitea.repository }}:latest
+          tags: ${{ vars.REGISTRY_URL }}/${{ gitea.repository }}:${{ steps.image-changes.outputs.image_tag }}
           context: "{{defaultContext}}:.docker"
+    outputs:
+      image_tag: ${{ steps.image-changes.outputs.image_tag }}
 
   build:
     runs-on: brutalisk