Update .NET SDK and container images

Configure dependabot to update the .NET 8 SDK in global.json and in the Dockerfile.

Author: martincostello

.github/dependabot.yml

@@ -1,5 +1,25 @@
 version: 2
 updates:
+  - package-ecosystem: "docker"
+    directory: "examples/net8.0/aspnetcore"
+    schedule:
+      interval: "weekly"
+      day: "wednesday"
+    ignore:
+      - dependency-name: "*"
+        update-types:
+          - "version-update:semver-major"
+          - "version-update:semver-minor"
+  - package-ecosystem: "dotnet-sdk"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "wednesday"
+    ignore:
+      - dependency-name: "*"
+        update-types:
+          - "version-update:semver-major"
+          - "version-update:semver-minor"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: