[change] Upgraded OpenVPN base image to lisenet/openvpn:2.6.17 #566

Changed the base image in order to use the latest OpenVPN version available
as the old base image used  was unmaintained for over 6 years.

Related to #566

Author: pandafy

images/openwisp_dashboard/openvpn.json

@@ -11,7 +11,16 @@
       "local": "",
       "comp_lzo": "no",
       "auth": "SHA1",
-      "data_ciphers": "AES-128-GCM:none",
+      "data_ciphers": [
+          {
+              "cipher": "AES-128-GCM",
+              "optional": false
+          },
+          {
+              "cipher": "none",
+              "optional": false
+          }
+      ],
       "data_ciphers_fallback": "AES-128-GCM",
       "cipher": "AES-128-GCM",
       "engine": "",

images/openwisp_openvpn/Dockerfile

@@ -1,5 +1,5 @@
 # hadolint ignore=DL3007
-FROM kylemanna/openvpn:2.4
+FROM lisenet/openvpn:2.6.17
 
 # hadolint ignore=DL3018
 RUN apk add --no-cache \