Dockerfile: run entrypoint as root

[babnskidata]

Hi!

From the commit message:

Since 492b2c56abfc, the entrypoint is executed as 'builder' user. 
Therefore, using a USER_ID and GROUP_ID currently fails: 
groupmod: Permission denied.
groupmod: cannot lock /etc/group; try again later. 
usermod: group '1000' does not exist
chown: changing ownership of '/builder/.bash_logout': Operation not permitted
chown: changing ownership of '/builder/.profile': Operation not permitted
chown: changing ownership of '/builder/.bashrc': Operation not permitted
chown: changing ownership of '/builder': Operation not permitted
error: failed switching to "builder": operation not permitted

As the entrypoint is designed to be executed as root, revert it back.

Reproducible with: docker run --rm -it -e "USER_ID=1000" -e "GROUP_ID=1000" ghcr.io/siemens/kas/kas:4.0
Debuggable with:

docker run --rm -it --entrypoint bash ghcr.io/siemens/kas/kas:4.0
builder@49a8a8928466:/$ USER_ID=1000 GROUP_ID=1000 /kas/container-entrypoint

[jan-kiszka]

Would break kas-container as discussed in https://groups.google.com/d/msgid/kas-devel/18e62f0d-2125-4cd5-ffd0-ef2b17246611%40siemens.com.