ci: build-oci-image: cache the docker image build

This wont always speed things up, but occasionally it will speed things
up by a lot

.gitea/workflows/gentoo-utils.yml

@@ -61,6 +61,8 @@ jobs:
       - name: Set up Docker buildx
         if: steps.image-changes.outputs.build_image == 'true'
         uses: docker/setup-buildx-action@v3
+        with:
+          driver-opts: network=runners-net
 
       - name: Log in to Github Container Registry
         if: steps.image-changes.outputs.build_image == 'true'
@@ -77,6 +79,8 @@ jobs:
           push: true
           tags: ${{ vars.REGISTRY_URL }}/${{ gitea.repository }}:${{ steps.image-changes.outputs.image_tag }}
           context: "{{defaultContext}}:.docker"
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
     outputs:
       image_tag: ${{ steps.image-changes.outputs.image_tag }}