Kokoro Ubuntu: Switch to docker image

presubmit.sh now runs presubmit-docker.sh using the new radial
docker image which contains various toolchains.

The `/bin/using.sh` bash script exports the `using` bash function
which can be called to configure toolchains at specific versions.
diff --git a/kokoro/ubuntu/presubmit-docker.sh b/kokoro/ubuntu/presubmit-docker.sh
new file mode 100755
index 0000000..82c64f0
--- /dev/null
+++ b/kokoro/ubuntu/presubmit-docker.sh
@@ -0,0 +1,39 @@
+#!/bin/bash
+
+set -e # Fail on any error.
+
+. /bin/using.sh # Declare the bash `using` function for configuring toolchains.
+
+set -x # Display commands being run.
+
+cd github/cppdap
+
+git submodule update --init
+
+if [ "$BUILD_SYSTEM" == "cmake" ]; then
+    using cmake-3.17.2
+    using gcc-9
+
+    mkdir build
+    cd build
+
+    build_and_run() {
+        cmake .. -DCPPDAP_BUILD_EXAMPLES=1 -DCPPDAP_BUILD_TESTS=1 -DCPPDAP_WARNINGS_AS_ERRORS=1 $1
+        make --jobs=$(nproc)
+
+        ./cppdap-unittests
+    }
+
+    if [ "$BUILD_SANITIZER" == "asan" ]; then
+        build_and_run "-DCPPDAP_ASAN=1"
+    elif [ "$BUILD_SANITIZER" == "msan" ]; then
+        build_and_run "-DCPPDAP_MSAN=1"
+    elif [ "$BUILD_SANITIZER" == "tsan" ]; then
+        build_and_run "-DCPPDAP_TSAN=1"
+    else
+        build_and_run
+    fi
+else
+    echo "Unknown build system: $BUILD_SYSTEM"
+    exit 1
+fi
\ No newline at end of file
diff --git a/kokoro/ubuntu/presubmit.sh b/kokoro/ubuntu/presubmit.sh
index 402b814..b69d40d 100755
--- a/kokoro/ubuntu/presubmit.sh
+++ b/kokoro/ubuntu/presubmit.sh
@@ -1,35 +1,16 @@
 #!/bin/bash
 
 set -e # Fail on any error.
-set -x # Display commands being run.
 
-BUILD_ROOT=$PWD
+ROOT_DIR=`pwd`
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd )"
 
-cd github/cppdap
-
-git submodule update --init
-
-if [ "$BUILD_SYSTEM" == "cmake" ]; then
-    mkdir build
-    cd build
-
-    build_and_run() {
-        cmake .. -DCPPDAP_BUILD_EXAMPLES=1 -DCPPDAP_BUILD_TESTS=1 -DCPPDAP_WARNINGS_AS_ERRORS=1 $1
-        make --jobs=$(nproc)
-
-        ./cppdap-unittests
-    }
-
-    if [ "$BUILD_SANITIZER" == "asan" ]; then
-        build_and_run "-DCPPDAP_ASAN=1"
-    elif [ "$BUILD_SANITIZER" == "msan" ]; then
-        build_and_run "-DCPPDAP_MSAN=1"
-    elif [ "$BUILD_SANITIZER" == "tsan" ]; then
-        build_and_run "-DCPPDAP_TSAN=1"
-    else
-        build_and_run
-    fi
-else
-    echo "Unknown build system: $BUILD_SYSTEM"
-    exit 1
-fi
\ No newline at end of file
+docker run --rm -i \
+  --volume "${ROOT_DIR}:${ROOT_DIR}" \
+  --volume "${KOKORO_ARTIFACTS_DIR}:/mnt/artifacts" \
+  --workdir "${ROOT_DIR}" \
+  --env BUILD_SYSTEM=$BUILD_SYSTEM \
+  --env BUILD_TARGET_ARCH=$BUILD_TARGET_ARCH \
+  --env BUILD_SANITIZER=$BUILD_SANITIZER \
+  --entrypoint "${SCRIPT_DIR}/presubmit-docker.sh" \
+  "gcr.io/shaderc-build/radial-build:latest"