configure dependabot to keep base image in dockerfile up-to-date

https://github.com/fphammerle/docker-onion-service/commit/c7831365777999b4f1908ed73ca77f40f1d2bb6f

.github/dependabot.yml

@@ -0,0 +1,12 @@
+# sync with https://github.com/fphammerle/docker-onion-service/blob/master/.github/dependabot.yml
+
+version: 2
+
+updates:
+- package-ecosystem: docker
+  directory: /
+  schedule:
+    interval: weekly
+    day: friday
+
+# https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/configuration-options-for-dependency-updates