The entrypoint script uses bash expressions, so should be run with /bin/bash.

pkg/docker/entrypoint.sh

@@ -1,8 +1,8 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 # This docker image supports sending either a flag or a command as the docker
-# command. When a flag is sent, it will be passed on to the metadata agent 
+# command. When a flag is sent, it will be passed on to the metadata agent
 # process. Anything else will be interpreted as the command to be run.
 #
 # Passing a flag.
@@ -14,6 +14,7 @@ set -e
 # Default behavior uses CMD defined in Dockerfile.
 # $ docker run -it {image:tag}
 
+# Note: substring substitution is a bash-ism.
 if [ "${1:0:1}" = '-' ]; then
   set -- /opt/stackdriver/metadata/sbin/metadatad "$@"
 fi