add presubmit_build.sh that passes a flag to swiftpm if it is a stock toolchain

docker/presubmit_build.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+
+set -exuo pipefail
+
+sudo apt-get install -y docker.io
+
+# Sets 'swift_tf_url' to the public url corresponding to
+# 'swift_tf_bigstore_gfile', if it exists.
+if [[ ! -z ${swift_tf_bigstore_gfile+x} ]]; then
+  export swift_tf_url="${swift_tf_bigstore_gfile/\/bigstore/https://storage.googleapis.com}"
+  case "$swift_tf_url" in
+    *stock*) STOCK_TOOLCHAIN=YES ;;
+    *) ;;
+  esac
+fi
+
+# Help debug the job's disk space.
+df -h
+
+# Move docker images into /tmpfs, where there is more space.
+sudo /etc/init.d/docker stop
+sudo mv /var/lib/docker /tmpfs/
+sudo ln -s /tmpfs/docker /var/lib/docker
+sudo /etc/init.d/docker start
+
+# Help debug the job's disk space.
+df -h
+
+cd github/swift-models
+sudo -E docker build -t built-img -f docker/Dockerfile --build-arg swift_tf_url .
+
+# SwiftPM-based build.
+docker run built-img /bin/bash -c "
+swift build ${STOCK_TOOLCHAIN:+"-Xswiftc -D -Xswiftc TENSORFLOW_USE_STANDARD_TOOLCHAIN"} ;
+swift test ${STOCK_TOOLCHAIN:+"-Xswiftc -D -Xswiftc TENSORFLOW_USE_STANDARD_TOOLCHAIN"} ;
+"
+
+# CMake-based build.
+sudo docker run built-img /bin/bash -c "
+set -e;
+cmake -B /BinaryCache/tensorflow-swift-models -D CMAKE_BUILD_TYPE=Release -D CMAKE_Swift_COMPILER=/swift-tensorflow-toolchain/usr/bin/swiftc -G Ninja -S /swift-models;
+cmake --build /BinaryCache/tensorflow-swift-models --verbose;
+"