Added -v arg to example build.py call (#146)

container/README.md

diff --git a/container/README.md b/container/README.md
index 5b1c64d..f20fb76 100644
--- a/container/README.md
+++ b/container/README.md
@@ -32,7 +32,7 @@
 the base container by running:
 
 ``` shell
-python container/build.py -p my-project-id -d rbe-ubuntu16_04 -c test-rbe-ubuntu16_04 -t latest -b my-gcs-bucket
+python container/build.py -p my-project-id -d rbe-ubuntu16_04 -c test-rbe-ubuntu16_04 -t latest -b my-gcs-bucket -v 0.15.2
 ```
 
 Congratulations! Your docker container is now available in [Container