installed easy-rsa for rasbian

openvpn/Dockerfile

 ARG CONFIG_DIR=/etc/default/config-files/
 
 RUN apt-get update &&\
-    apt-get install -y ssmtp openvpn openvpn-auth-ldap openvpn-auth-radius ldap-utils &&\
+    apt-get install -y ssmtp openvpn openvpn-auth-ldap openvpn-auth-radius ldap-utils easy-rsa &&\
     apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* && \
     rm /etc/cron.*/*