perf: use BUILDPLATFORM for Go builder stage to avoid QEMU emulation

Without --platform=$BUILDPLATFORM, the golang builder image is pulled
for the target arch (arm64) and run via QEMU on the amd64 runner,
making builds extremely slow (~25min). With BUILDPLATFORM, the builder
always runs natively on the host arch while cross-compiling via GOARCH.
Build time should drop from ~25min to ~3-5min.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: decobot

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM golang:1.24 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.24 AS builder
 ARG TARGETOS
 ARG TARGETARCH