upgrade base image alpine v3.9 -> v3.12; pin openvpn package version

Dockerfile

@@ -1,6 +1,7 @@
-FROM alpine:3.9
+FROM alpine:3.12
 
-RUN apk add --no-cache openvpn
+ARG OPENVPN_PACKAGE_VERSION=2.4.9-r0
+RUN apk add --no-cache openvpn=$OPENVPN_PACKAGE_VERSION
 
 VOLUME /vpn-config
 COPY ./vpn.sh /vpn.sh