toolbox: install ping

After updating toolbox docker base image,
ping is not installed by default. This CL
updates the Dockerfile to install ping.

BUG= b/239857536
TEST=run `toolbox ping` in a COS dev instantce.

Change-Id: I60c3cb9261d821cc2c183bb3a8e3d2ea3e244d75
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/35028
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: He Gao <hegao@google.com>
diff --git a/src/cmd/toolbox/Dockerfile b/src/cmd/toolbox/Dockerfile
index 5475a9b..eec4a37 100644
--- a/src/cmd/toolbox/Dockerfile
+++ b/src/cmd/toolbox/Dockerfile
@@ -26,7 +26,7 @@
 # Various networking and other tools. net-tools installs arp, netstat, etc.
 RUN apt-get install -u -qq vim \
     net-tools netcat ipset conntrack inetutils-traceroute bridge-utils \
-    ebtables \
+    ebtables iputils-ping \
     && apt-get clean
 
 # These packages are required or extracting source tarballs and building the kernel.