Update Dependabot config for Docker (#1936)

Since it appears the docker updater doesn't traverse the repo
and needs the specific directory containing the `Dockerfile`.

Fixes:
https://github.com/heroku/heroku-buildpack-python/network/updates/1118250366

GUS-W-19898523.

Author: edmorley

.github/dependabot.yml

@@ -14,7 +14,7 @@ updates:
           - "minor"
           - "patch"
   - package-ecosystem: "docker"
-    directory: "/"
+    directory: "/builds"
     schedule:
       interval: "monthly"
     labels: