Bump docker/build-push-action in /.github/workflows (#52669)

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 6.13.0 to 6.15.0.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@ca877d9...471d1dc)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/build-ci-service-images.yaml

@@ -44,7 +44,7 @@ jobs:
 
       - name: Build etcd image
         id: docker_build
-        uses: docker/build-push-action@ca877d9245402d1537745e0e356eab47c3520991 # v6.13.0
+        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4 # v6.15.0
         with:
           context: ${{ github.workspace }}
           file: .github/services/Dockerfile.etcd

.github/workflows/build-usage-image.yaml

@@ -24,7 +24,7 @@ jobs:
         with:
           registry-type: public
       # Build and publish container image on ECR.
-      - uses: docker/build-push-action@ca877d9245402d1537745e0e356eab47c3520991 # v6.13.0
+      - uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4 # v6.15.0
         with:
           context: "examples/teleport-usage"
           tags: public.ecr.aws/gravitational/teleport-usage:${{ steps.version.outputs.version }}