Update swift_tf_url to use x10 (#437)

Author: asuhan

docker/Dockerfile

@@ -1,7 +1,7 @@
 FROM gcr.io/swift-tensorflow/base-deps-cuda10.1-cudnn7-ubuntu18.04
 
 # Allows the caller to specify the toolchain to use.
-ARG swift_tf_url=https://storage.googleapis.com/swift-tensorflow-artifacts/nightlies/latest/swift-tensorflow-DEVELOPMENT-ubuntu18.04.tar.gz
+ARG swift_tf_url=https://storage.googleapis.com/swift-tensorflow-artifacts/nightlies/latest/swift-tensorflow-DEVELOPMENT-x10-ubuntu18.04.tar.gz
 
 # Download and extract S4TF
 WORKDIR /swift-tensorflow-toolchain