Add a new branch cut GH workflow, and adopt setuptools-scm for version control (#15985)

docker/Dockerfile

@@ -12,7 +12,7 @@ ARG HOPPER_SBO_DEEPEP_COMMIT=9f2fc4b3182a51044ae7ecb6610f7c9c3258c4d6
 ARG DEEPEP_COMMIT=9af0e0d0e74f3577af1979c9b9e1ac2cad0104ee
 ARG BUILD_AND_DOWNLOAD_PARALLEL=8
 ARG SGL_KERNEL_VERSION=0.3.20
-ARG SGL_VERSION=0.5.6.post2
+ARG SGL_VERSION
 ARG USE_LATEST_SGLANG=0
 ARG GDRCOPY_VERSION=2.5.1
 ARG PIP_DEFAULT_INDEX
@@ -183,6 +183,8 @@ RUN if [ "$BRANCH_TYPE" = "local" ]; then \
         cp -r /tmp/local_src /sgl-workspace/sglang; \
     elif [ "$USE_LATEST_SGLANG" = "1" ]; then \
         git clone --depth=1 https://github.com/sgl-project/sglang.git /sgl-workspace/sglang; \
+    elif [ -z "$SGL_VERSION" ]; then \
+        echo "ERROR: SGL_VERSION must be set when USE_LATEST_SGLANG=0 and BRANCH_TYPE!=local" && exit 1; \
     else \
         git clone --depth=1 --branch v${SGL_VERSION} https://github.com/sgl-project/sglang.git /sgl-workspace/sglang; \
     fi \