build: route docker-build through buildx for multi-arch + push

Setting REGISTRY now switches `make docker-build` from a single-arch
local build to a multi-arch (linux/amd64+linux/arm64) buildx build
that pushes $REGISTRY/$IMAGE:$VERSION and :latest. The redundant
docker-buildx target is removed. Bump the builder base image to
golang:1.25-alpine so protoc-gen-go-grpc@latest installs cleanly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM golang:1.24-alpine AS builder
+FROM golang:1.25-alpine AS builder
 
 # Build arguments
 ARG VERSION=dev