kokoro: run tests with a default docker container runtime

We want to know that our environment set up properly
and docker tests pass with a native runtime.

PiperOrigin-RevId: 248229294
Change-Id: I06c221e5eeed6e01bdd1aa935333c57e8eadc498

kokoro/run_tests.sh

@@ -142,10 +142,21 @@ EOF
 run_docker_tests() {
   cd ${WORKSPACE_DIR}
 
+  # Run tests with a default runtime (runc).
+  bazel test \
+    "${BAZEL_BUILD_FLAGS[@]}" \
+    --test_env=RUNSC_RUNTIME="" \
+    --test_output=all \
+    //runsc/test/image:image_test
+
   # These names are used to exclude tests not supported in certain
   # configuration, e.g. save/restore not supported with hostnet.
   declare -a variations=("" "-kvm" "-hostnet" "-overlay")
   for v in "${variations[@]}"; do
+    # FIXME(b/132073574): we need to flush arp tables, otherwise tests fail with
+    # timeout.
+    sudo ip neigh show
+    sudo ip neigh flush dev docker0
     # Run runsc tests with docker that are tagged manual.
     bazel test \
       "${BAZEL_BUILD_FLAGS[@]}" \

runsc/test/testutil/docker.go

@@ -31,8 +31,8 @@ import (
 )
 
 func getRuntime() string {
-	r := os.Getenv("RUNSC_RUNTIME")
-	if r == "" {
+	r, ok := os.LookupEnv("RUNSC_RUNTIME")
+	if !ok {
 		return "runsc-test"
 	}
 	return r