Fix for #174: Incrase healthcheck timeout

Default timeout for healthcheck by kubernetes is too low. As we wait up
for 2 seconds for the Postfix and DKIM to respond, this exceeds the
threshold and the healthcheck fails with:

```
Readiness probe failed: command "sh -c /scripts/healthcheck.sh" timed out
```

By simply increasing `timeoutSeconds` this issue is resolved.

helm/mail/values.yaml

@@ -205,6 +205,7 @@ readinessProbe:
   initialDelaySeconds: 10
   periodSeconds: 60
   failureThreshold: 6
+  timeoutSeconds: 8
   exec:
     command:
       - sh

scripts/healthcheck.sh

@@ -18,5 +18,8 @@ check_dkim() {
     printf '\x18Clocalhost\x004\x00\x00127.0.0.1\x00' | nc -w 2 127.0.0.1 8891
 }
 
+echo "Postfix check..."
 check_postfix
+echo "DKIM check..."
 check_dkim
+echo "All OK!"