Use custom Dockerfile with node and pip

.forgejo/workflows/mkdocs_publish.yaml

@@ -16,8 +16,9 @@ env:
 jobs:
   build:
     runs-on: docker
+    container:
+        image: git.lordwektabyte.cat/lordwektabyte/node-pip:latest
     steps:
-      - run: apt-get update && apt-get install python3-pip -y
       - uses: https://code.forgejo.org/actions/checkout@v4
         with:
           fetch-depth: 0