needed for insecure key usage

`chmod 600` is for the benefit of ssh client, which will otherwise spew:

```
chmod 600 image/insecure_key.pub
SSHing into 172.17.0.2
Warning: Permanently added '172.17.0.2' (ECDSA) to the list of known hosts.
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0664 for 'image/insecure_key' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
bad permissions: ignore key: image/insecure_key
```

As you can see though, the wrong file is `chmod`'ed -- it's the private key which ssh client will insist should be protected with a more restrictive mode.

After this, `CMD ["/sbin/my_init", "--enable-insecure-key"]` and `make ssh` works as expected.