Use non-privileged user in Dockerfile

[Doctor-love]

Currently, the Dockerfile used to build the official Docker images runs geth as root.
This is not required as far as I can tell from my usage and not a good practice security wise.

I suggest using the Dockerfile directive "USER" or switching to a low privileged user using gosu.

[alexkohler]

Wouldn't the package management (i.e. apk add) still require root permissions?

[Doctor-love]

Privileges could be dropped after the package installation and setup has been completed

[jeanlucmongrain]

This issue had been """"""""""""""fixed"""""""""""""" in the worst possible way issue should be reopen

[jeanlucmongrain]

#16052 (comment)

this task should be reopen