USER $HOME uses uid instead of username

[hobti01]

It seems that $HOME is set to the value of the USER instruction.

This causes problems with OpenShift compatible images

the final USER declaration in the Dockerfile should specify the user ID (numeric value) and not the user name

Using the uid causes unexpected behaviour:

USER 1000

RUN touch ~/invalid

output

INFO[0051] cmd: /bin/sh                                 
INFO[0051] args: [-c touch ~/invalid]                   
touch: cannot touch '/home/1000/invalid': No such file or directory
Error: error building image: waiting for process to exit: exit status 1

Workaround is to set $HOME explicitly with ARG or ENV, but it would be great if $HOME was resolved to the users $HOME.

Thanks for the great work!