upgrade openvpn package from v2.5.4-r0 to v2.5.6-r0 (see below)

https://web.archive.org/web/20220325201851/https://github.com/OpenVPN/openvpn/blob/v2.5.6/ChangeLog

https://git.alpinelinux.org/aports/commit/main/openvpn?h=3.15-stable&id=fa2301d59143e83bc65bdacf50a6765d35020a01
https://git.alpinelinux.org/aports/commit/main/openvpn?h=3.15-stable&id=7cbcdee99932e88c06f67e361c6c30c04e85ef0f

Dockerfile

@@ -1,6 +1,6 @@
 FROM docker.io/alpine:3.15.1
 
-ARG OPENVPN_PACKAGE_VERSION=2.5.4-r0
+ARG OPENVPN_PACKAGE_VERSION=2.5.6-r0
 RUN apk add --no-cache openvpn=$OPENVPN_PACKAGE_VERSION
 
 VOLUME /vpn-config