add docker to dependabot

This keeps our Dockerfiles up to date along with everything else.

It won't be perfect (swapping from bookworm to trixie will need a manual
bump), but it gets the basics done.

.github/dependabot.yml

@@ -10,4 +10,14 @@ updates:
         applies-to: version-updates
         patterns:
           - "github.com/aws/aws-sdk-go-v2/*"
-          - "github.com/aws/aws-sdk-go-v2"
+          - "github.com/aws/aws-sdk-go-v2"
+- package-ecosystem: "docker"
+  directories:
+    - "/"
+    - "database"
+    - "tools/autograph-monitor"
+    - "tools/softhsm"
+    - "tools/config-sanitizer"
+  schedule:
+    interval: "weekly"
+    time: "10:00" # UTC