fix(ci): fix dependabot docker action

Dependabot already searches for any files matching
`f.match?(/dockerfile/i)` within the given directory, we don't need to
give it hints about filenames

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>

.github/dependabot.yml

@@ -2,7 +2,7 @@
 version: 2
 updates:
   - package-ecosystem: docker
-    directory: "/Dockerfile.*"
+    directory: /
     schedule:
       interval: daily
     labels: