deploy: b300 build scripts with pip cache mount + no-build-isolation fast path

Base image already has torch 2.13, but pyproject build-system.requires
pulls it into pip's isolated build env on every build (~6GB via throttled
mirror, 47min). --no-build-isolation reuses the base env; build.sh picks
Dockerfile.fast (--no-deps) when python/pyproject.toml is unchanged.

deploy/b300-build/Dockerfile.fast

@@ -0,0 +1,15 @@
+# fast 路径：python/pyproject.toml 未变化时使用（build.sh 自动判断）。
+# --no-deps 跳过全部依赖解析（连 index 元数据请求都省掉），纯源码迭代秒级完成。
+# 依赖有变化时必须走 Dockerfile 全量路径（build.sh 按 pyproject sha256 自动切换）。
+FROM uhub.service.ucloud.cn/umirror/sglang:dev-dsv41
+
+ARG PIP_INDEX=https://pypi.tuna.tsinghua.edu.cn/simple
+ENV PIP_INDEX_URL=${PIP_INDEX}
+
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install "setuptools>=61" "setuptools-scm>=8" "setuptools-rust>=1.11" wheel
+
+COPY sglang/ /sgl-workspace/sglang/
+
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install --no-deps --no-build-isolation -e /sgl-workspace/sglang/python