Update entrypoint in dockerfile for vmpooler gem

This commit updates dockerfile entrypoint to remove the explicit
vmpooler executable path. Additionally, CMD is added to run a default
command when runtime parameters are not passed in by the user. Without
this change the vmpooler executable path is hardcoded into a directory
that will not exist.

docker/docker-entrypoint.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 set -e
 
-set -- /var/lib/vmpooler/vmpooler "$@"
+set -- vmpooler "$@"
 
 exec "$@"