Update Kokoro image to install Golang 1.13

PiperOrigin-RevId: 284308422

kokoro/ubuntu1604/10_core.sh

@@ -21,8 +21,8 @@ apt-get update && apt-get -y install make git-core build-essential linux-headers
 
 # Install a recent go toolchain.
 if ! [[ -d /usr/local/go ]]; then
-    wget https://dl.google.com/go/go1.12.linux-amd64.tar.gz
-    tar -xvf go1.12.linux-amd64.tar.gz
+    wget https://dl.google.com/go/go1.13.5.linux-amd64.tar.gz
+    tar -xvf go1.13.5.linux-amd64.tar.gz
     mv go /usr/local
 fi
 

kokoro/ubuntu1604/README.md

@@ -0,0 +1,34 @@
+## Image Update
+
+After making changes to files in the directory, you must run the following
+commands to update the image Kokoro uses:
+
+```shell
+gcloud config set project gvisor-kokoro-testing
+third_party/gvisor/kokoro/ubuntu1604/build.sh
+third_party/gvisor/kokoro/ubuntu1804/build.sh
+```
+
+Note: the command above will change your default project for `gcloud`. Run
+`gcloud config set project` again to revert back to your default project.
+
+Note: Files in `third_party/gvisor/kokoro/ubuntu1804/` as symlinks to
+`ubuntu1604`, therefore both images must be updated.
+
+After the script finishes, the last few lines of the output will container the
+image name. If the output was lost, you can run `build.sh` again to print the
+image name.
+
+```
+NAME                    PROJECT                FAMILY  DEPRECATED  STATUS
+image-6777fa4666a968c8  gvisor-kokoro-testing                      READY
++ cleanup
++ gcloud compute instances delete --quiet build-tlfrdv
+Deleted [https://www.googleapis.com/compute/v1/projects/gvisor-kokoro-testing/zones/us-central1-f/instances/build-tlfrdv].
+```
+
+To setup Kokoro to use the new image, copy the image names to their
+corresponding file below:
+
+*   //devtools/kokoro/config/gcp/gvisor/ubuntu1604.gcl
+*   //devtools/kokoro/config/gcp/gvisor/ubuntu1804.gcl

scripts/go.sh

@@ -25,6 +25,8 @@ tools/go_branch.sh
 # Checkout the new branch.
 git checkout go && git clean -f
 
+go version
+
 # Build everything.
 go build ./...