create_images.py: Use short Git hashs in the image name

buildkite/create_images.py

diff --git a/buildkite/create_images.py b/buildkite/create_images.py
index 1c7c807..1d3041a 100755
--- a/buildkite/create_images.py
+++ b/buildkite/create_images.py
@@ -221,7 +221,7 @@
         argv = sys.argv[1:]
 
     try:
-        git_commit = subprocess.check_output(['git', 'rev-parse', '--verify', 'HEAD'],
+        git_commit = subprocess.check_output(['git', 'rev-parse', '--verify', '--short', 'HEAD'],
                                              universal_newlines=True).strip()
     except subprocess.CalledProcessError:
         print("Could not get current Git commit hash. You have to run "