Fix Makefile: Add build args to docker-buildx target

[carlory]

Dockerfile.cross:5
--------------------
   3 |
   4 |     # Build the manager binary
   5 | >>> FROM --platform=${BUILDPLATFORM} ${BUILDER_IMAGE} as builder
   6 |     ARG TARGETOS
   7 |     ARG TARGETARCH
--------------------
ERROR: failed to solve: base name (${BUILDER_IMAGE}) should not be blank

Test with this PR:

(base) ➜  scheduler-plugins git:(isvc) IMG=ghcr.io/carlory/inftyai/scheduler-plugins make docker-buildx
Makefile:205: warning: overriding commands for target `/Users/kiki/workspace/golang/src/github.com/inftyai/scheduler-plugins/bin'
Makefile:44: warning: ignoring old commands for target `/Users/kiki/workspace/golang/src/github.com/inftyai/scheduler-plugins/bin'
# copy existing Dockerfile and insert --platform= into Dockerfile.cross, and preserve the original Dockerfile
sed -e '1 s/\(^FROM\)/FROM --platform=\$\{BUILDPLATFORM\}/; t' -e ' 1,// s//FROM --platform=\$\{BUILDPLATFORM\}/' Dockerfile > Dockerfile.cross
docker buildx create --name project-v3-builder
project-v3-builder
docker buildx use project-v3-builder
docker buildx build --push --platform=linux/arm64,linux/amd64,linux/s390x,linux/ppc64le --tag ghcr.io/carlory/inftyai/scheduler-plugins --build-arg BASE_IMAGE=gcr.io/distroless/static:nonroot --build-arg BUILDER_IMAGE=golang:1.24.1 --build-arg CGO_ENABLED=  -f Dockerfile.cross .
[+] Building 1142.1s (44/44) FINISHED                                                               docker-container:project-v3-builder
...
docker buildx rm project-v3-builder
project-v3-builder removed
rm Dockerfile.cross

[kerthcet]

/retest

[kerthcet]

/lgtm
/approve
/kind cleanup