Configure dependabot to update Docker images

This syncs this repository's dependabot configuration up with the one
from Memray.

Signed-off-by: Matt Wozniski <godlygeek@gmail.com>

Author: godlygeek

.github/dependabot.yml

@@ -4,3 +4,11 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    cooldown:
+      default-days: 7
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    cooldown:
+      default-days: 7