build(deps): Bump docker/build-push-action from 5 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 5 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v5...v6)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -70,7 +70,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
 
       - name: Build KDC image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: "{{defaultContext}}:testdata"
           load: true
@@ -80,7 +80,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Build DNS image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: "{{defaultContext}}:testdata"
           load: true
@@ -90,7 +90,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Extract keytab
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: "{{defaultContext}}:testdata"
           outputs: type=local,dest=testdata