Update Kaniko executor image in Docker workflow

Replaced the Kaniko executor image with a self-hosted version to improve control and reliability. This ensures the workflow uses a specific version (`25.21.1`) hosted on `git.van-hemmen.com`.

.forgejo/workflows/docker-tag.yaml

@@ -6,7 +6,7 @@ jobs:
   docker-tag:
     runs-on: docker
     container:
-      image: gcr.io/kaniko-project/executor:debug
+      image: git.van-hemmen.com/actions/kaniko:25.21.1
     steps:
       - name: Build & push with Kaniko
         run: /bin/build.sh