Add Dockerfile for Dependabot (#525)

.github/dependabot.yml

@@ -8,7 +8,11 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-  - package-ecosystem: gomod
+  - package-ecosystem: "gomod"
     directory: "/"
     schedule:
-      interval: weekly
+      interval: "weekly"
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"