Thanks for the hint. I changed

Copy newandroid.img to /home/phablet/anbox-data/

to

Copy newandroid.img to /home/phablet/anbox-data/ on the phone.

That should make it clear.

There it is

Thank you @Ingo. I've added it to the code and it works.
As this is the reboot approach I'd still leave the issue open as maybe someone might come up with the preferred refresh solution.

Thanks for the information / advice.