add volume to persist created short links

compose.yml

@@ -6,6 +6,8 @@ services:
     image: shlinkio/shlink:4.4
     networks:
       - proxy
+    volumes:
+      - shlink_data:/etc/shlink
     deploy:
       restart_policy:
         condition: on-failure
@@ -20,3 +22,5 @@ services:
 networks:
   proxy:
     external: true
+volumes:
+  shlink_data: