ci: remove docker ecosystem from dependabot

The devcontainer Go base image version must match go.mod. Dependabot treats the image tag as semver and proposes invalid bumps (e.g. go:1.25 to go:2.1). The devcontainer-check.yml CI workflow already guards against version mismatches, so automated Dependabot docker updates add no value here.

Author: dlevy-msft-sql

.github/dependabot.yml

@@ -42,16 +42,3 @@ updates:
       github-actions:
         patterns:
           - "*"
-
-  # Enable version updates for Dockerfile base images
-  - package-ecosystem: "docker"
-    directory: "/.devcontainer"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-    labels:
-      - "dependencies"
-      - "devcontainer"
-    commit-message:
-      prefix: "deps"
-      include: "scope"