ci: switch build from kaniko to docker buildx with registry cache

- build step: kaniko -> woodpeckerci/plugin-docker-buildx
- enable registry layer cache (cache_images) for fast rebuilds
- keep extra_hosts to push over internal network
- Dockerfile: BuildKit pip cache mount + syntax/env tidy

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -1,17 +1,14 @@
+# syntax=docker/dockerfile:1
 FROM python:3.10-slim
 
-#
+ENV PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1
+
 WORKDIR /code
 
-#
 COPY ./requirements.txt /code/requirements.txt
 
-#
-RUN pip install --no-cache-dir --upgrade -r /code/requirements.txt -i https://pypi.tuna.tsinghua.edu.cn/simple
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install --upgrade -r /code/requirements.txt -i https://pypi.tuna.tsinghua.edu.cn/simple
 
-#
 COPY ./app /code/app
-
-
-#
-# CMD ["python", "/code/app/demo.py"]