GCE - Can not create an instance image from the startup script - rsync

Question

I am trying to create a GCE instance image from its startup script (just after some setup instructions). I use a command line looking like :
gcimagebundle -d "/dev/sda" -o "/tmp/" --log_file=/var/log/image_generation.log "--output_file_name=/tmp/myimage.tar.gz"
After few seconds the gcimagebundle exits and I get the following error message (from the /var/log/image_generation.log file) :
INFO:root:found platform Centos
INFO:root:File System: ext4
INFO:root:Disk Size: 10737418240 bytes
INFO:root:Resolved /dev/sda to /dev/sda
INFO:root:exclude list: (/etc/ssh/.host_key_regenerated, 0:0:0) (/dev, 0:1:0) (/proc, 0:1:0) (/run, 0:1:1) (/selinux, 0:0:0) (/tmp, 0:1:0) (/sys, 0:1:0) (/var/lib/google/per- instance, 0:1:0) (/var/lock, 0:1:1) (/var/log, 0:1:1) (/var/run, 0:1:1)
INFO:root:ignoring mounts /proc /sys /dev/pts /dev/shm /proc/sys/fs/binfmt_misc
WARNING:root:overwrite list =
INFO:root:Initializing disk file
INFO:root:Making filesystem
INFO:root:Copying contents
WARNING:root:Error while running ['rsync', '--times', '--perms', '--owner', '--group', '--links', '--devices', '--acls', '--sparse', '--hard-links', '--recursive', '--xattrs', '--exclude-from=/tmp/tmpXtECHY/tmp6Y_7Dr', '/', '/tmp/tmpXtECHY/tmpencYI_'] return_code = 11
stdout=
stderr=rsync: failed to open exclude file /tmp/tmpXtECHY/tmp6Y_7Dr: Permission denied (13)
rsync error: error in file IO (code 11) at exclude.c(1062) [client=3.0.6]
I tried to execute the same startup script on a fresh instance after a manual sudo su and all works.
I execute a whoami from the startup script and the result is root.