new: [drone] add ``backup`` relation

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

drone/metadata.yml

@@ -47,3 +47,17 @@ uses:
       proxy: "Public access"
     default-options:
       target: !var-expand ${MASTER_BASE_SERVICE_NAME}:80
+  backup:
+    constraint: recommended
+    auto: pair
+    solves:
+      backup: "Automatic regular backups of sql database"
+    default-options:
+      ## First pattern matching wins, no pattern matching includes.
+      ## include-patterns are checked first, then exclude-patterns
+      ## Patterns rules:
+      ##  - ending / for directory
+      ##  - '*' authorized
+      ##  - must start with a '/', will start from $SERVICE_DATASTORE
+      # include-patterns:
+      #   - /var/backups/pg/