Fix some /etc/ file permissions (see containerd#2684)

[apostasie]

@AkihiroSuda @rafi-gh

This intend to fix #2684

From local testing with a 0077 umask:

apo@oliphant:~ $ sudo ./nerdctl run -ti --network host --rm debian ls -lA
-rw------- 1 root root 241 May 10 15:28 /etc/resolv.conf
apo@oliphant:~ $ sudo ./nerdctl-patch run -ti --network host --rm debian ls -lA /etc/resolv.conf
-rw-r--r-- 1 root root 241 May 10 15:28 /etc/resolv.conf

The patch is very simple, and just chmod all three files after writing.

Let me know if this is acceptable in this state or if a more involved refactoring is in order.

[apostasie]

CI is happy.
PTAL.

Thanks!

[AkihiroSuda]

The commit looks good, but it's title should be more descriptive

[apostasie]

Fixed. Let me know if this is better.

Thank you @AkihiroSuda

[apostasie]

Not sure what the CI failure is about.
Previous run was fine.
Is this flaky in any way? Or did I actually mess something here?

[AkihiroSuda]

Thanks