Remove USER directive

I forgot that not only does USER apply to the process launched inside
the container, it also applies to any directives later in the build
process, including those for images that inherit FROM this image.  As
such, those images would have to have a USER directive before running
anything as root, e.g. `dnf install`, and then another USER directive to
set the process user.  Since this image isn't intended to be used
directly there's really no point in setting USER here, then.

Containerfile

@@ -5,6 +5,4 @@ RUN groupadd -g 1000 jenkins \
 
 COPY known-hosts-command.ssh.conf /etc/ssh/ssh_config.d/10-known-hosts-command.conf
 
-USER jenkins:jenkins
-
 CMD ["python3", "-c", "import signal;signal.pause()"]