configure dependabot to keep base image & github-actions up-to-date

https://github.com/fphammerle/docker-onion-service/commit/16856af330263d1331895212ae2e6927a35159a4
https://github.com/fphammerle/ical2vdir/commit/76d3b61c128fe1cad322e52d5877cc919a533ac4

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+
+updates:
+- package-ecosystem: docker
+  directory: /
+  schedule:
+    interval: weekly
+    day: friday
+- package-ecosystem: github-actions
+  directory: /
+  # > YAML aliases are not supported
+  schedule:
+    interval: weekly
+    day: friday
+
+# https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/configuration-options-for-dependency-updates