ci(dependabot): add dependabot docker ecosystem (#3885)

This means more dependabot PRs, and hopefully less manual work keeping Dockerfiles up to date.

In some cases careful testing will be required before merging Docker PRs but at least we have a to-do list in the form of dependabot PRs.

We do not use dependabot with npm yet because our build process is currently incompatible with "pure ESM build" dependencies.  See also  #3841

.github/dependabot.yml

@@ -5,9 +5,22 @@
 
 version: 2
 updates:
-  # Maintain dependencies for GitHub Actions
+  # Automatically update GitHub Actions
   - package-ecosystem: "github-actions"
+    directory: "/"
+    open-pull-requests-limit: 5
+    schedule:
+      interval: "weekly"
+  # Automatically update Dockerfile FROM images
+  - package-ecosystem: "docker"
     directory: "/"
     open-pull-requests-limit: 5
     schedule:
       interval: "daily"
+  # TODO: Consider enabling the NPM package ecosystem once we have fixed our build process to support pure ESM build dependencies, see also https://github.com/garden-io/garden/issues/3841
+  # Automatically update NPM packages
+  # - package-ecosystem: "npm"
+  #   directory: "/"
+  #   open-pull-requests-limit: 5
+  #   schedule:
+  #     interval: "daily"