fix(docker): cross-compile builder stage to avoid QEMU-emulated arm64 build

Pin the Go builder stage to $BUILDPLATFORM so arm64 binaries are produced by
native cross-compilation (GOARCH=$TARGETARCH) on the amd64 runner instead of
running the Go compiler under QEMU emulation. The emulated arm64 build took
~50 min and pushed the satellite image past the 60-minute release job timeout,
leaving blockstor-satellite unpublished for v0.1.0.

Signed-off-by: Andrei Kvapil <andrei.kvapil@aenix.io>
Co-Authored-By: Claude <noreply@anthropic.com>

Author: kvaps

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM golang:1.25 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.25 AS builder
 ARG TARGETOS
 ARG TARGETARCH