docker: Fix post function for Alpine build (package)

docker: Fix post function for Alpine build (package)

It was using a wrong sysdir.

```
--------------------
  72 |     # Own the config / PID files
  73 |     RUN mkdir -p /var/run/frr
  74 | >>> RUN chown -R frr:frr /etc/frr /var/run/frr
  75 |
  76 |     # Simple init manager for reaping processes and forwarding signals
--------------------
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>