Add Docker ecosystem to Dependabot config

Added Docker package ecosystem to Dependabot configuration.

Author: macbre

.github/dependabot.yml

@@ -7,5 +7,9 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      # Check for updates to GitHub Actions every weekday
+      interval: "daily"
+
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
       interval: "daily"