chore(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] <[email protected]>

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
 
       - name: Run unit tests
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: false
@@ -45,7 +45,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Run integration tests
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: false
@@ -63,7 +63,7 @@ jobs:
           password: ${{ secrets.DOCKER_TOKEN }}
 
       - name: Build dlang-tour runner image and push it
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: ${{ github.event_name == 'push' || github.event_name == 'schedule' }}