feat: ensure referenced images get updated by dependabot

Dokku uses a few external images for various bits of functionality, but these must be manually updated every so often due to being referenced in code.

By moving the references to Dockerfiles, we can take advantage of dependabot to issue automatic PRs when they are out of date.

.github/dependabot.yml

@@ -141,6 +141,41 @@ updates:
     open-pull-requests-limit: 2
     labels:
       - "type: dependencies"
+  - package-ecosystem: "docker"
+    directory: "/plugins/caddy-vhosts"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 10
+    labels:
+      - "type: dependencies"
+  - package-ecosystem: "docker"
+    directory: "/plugins/haproxy-vhosts"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 10
+    labels:
+      - "type: dependencies"
+  - package-ecosystem: "docker"
+    directory: "/plugins/logs"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 10
+    labels:
+      - "type: dependencies"
+  - package-ecosystem: "docker"
+    directory: "/plugins/openresty-vhosts"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 10
+    labels:
+      - "type: dependencies"
+  - package-ecosystem: "docker"
+    directory: "/plugins/traefik-vhosts"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 10
+    labels:
+      - "type: dependencies"
   - package-ecosystem: bundler
     directory: "/tests/apps/ruby"
     schedule: