Restrict permissions of private key file

The access rights of the private key file are restricted to the owner
when it is created. On existing installations this must be done
manually.

Author: siegfriedweber

CHANGELOG.adoc

@@ -5,6 +5,7 @@
 :224: https://github.com/stackabletech/agent/pull/224[#224]
 :229: https://github.com/stackabletech/agent/pull/229[#229]
 :234: https://github.com/stackabletech/agent/pull/234[#234]
+:235: https://github.com/stackabletech/agent/pull/235[#235]
 
 === Added
 * `hostIP` and `podIP` added to the pod status ({224}).
@@ -13,6 +14,10 @@
 === Fixed
 * Invalid or unreachable repositories are skipped when searching for
   packages ({229}).
+* Access rights of the private key file restricted to the owner ({235}).
+  The permissions are set when the file is created. On existing
+  installations the permissions must be set manually, e.g. with
+  `chmod 600 /etc/stackable/stackable-agent/secret/agent.key`.
 
 == 0.4.0 - 2021-06-23