docs: fix injecting root keys example

[runcom]

Fails with the following otherwise:

STEP 9/11: RUN mkdir -p /usr/etc-system/ &&     echo 'AuthorizedKeysFile /usr/etc-system/%u.keys' >> /etc/ssh/sshd_config.d/30-auth-system.conf &&     echo 'ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIL7xFq1HtZKZiaD8MfkhNtn37m8GSc1W168NoSaT9RSf cardno:000F_C36A3FC0' > /usr/etc-system/root.keys && chmod 0600 /usr/etc-system/keys
chmod: cannot access '/usr/etc-system/keys': No such file or directory
Error: building at STEP "RUN mkdir -p /usr/etc-system/ &&     echo 'AuthorizedKeysFile /usr/etc-system/%u.keys' >> /etc/ssh/sshd_config.d/30-auth-system.conf &&     echo 'ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIL7xFq1HtZKZiaD8MfkhNtn37m8GSc1W168NoSaT9RSf cardno:000F_C36A3FC0' > /usr/etc-system/root.keys && chmod 0600 /usr/etc-system/keys": while running runtime: exit status 1

[dougsland]

LGTM

[vrothberg]

LGTM