Add start.sh and Makefile build-only targets

start.sh builds via 'make build' (platform auto-detected) then starts
services detached with 'docker compose up -d'.

Makefile gains build/build-<platform> targets that build images without
starting, mirroring the existing up/<platform> targets.

infra/Makefile

@@ -10,10 +10,29 @@
 #   up-armv6                     — linux/arm/v6 (Pi Zero, Pi 1)
 
 COMPOSE = docker compose
+BUILD   = $(COMPOSE) build
 UP      = $(COMPOSE) up --build
 
-.PHONY: up up-amd64 up-x64 up-win up-arm64 up-armv7 up-armv6 down logs
+.PHONY: build build-amd64 build-x64 build-win build-arm64 build-armv7 build-armv6 \
+        up up-amd64 up-x64 up-win up-arm64 up-armv7 up-armv6 down logs
 
+# ── Build only (no start) ─────────────────────────────────────────────────────
+build:
+	$(BUILD)
+
+build-amd64 build-x64 build-win:
+	DOCKER_DEFAULT_PLATFORM=linux/amd64   $(BUILD)
+
+build-arm64:
+	DOCKER_DEFAULT_PLATFORM=linux/arm64   $(BUILD)
+
+build-armv7:
+	DOCKER_DEFAULT_PLATFORM=linux/arm/v7  $(BUILD)
+
+build-armv6:
+	DOCKER_DEFAULT_PLATFORM=linux/arm/v6  $(BUILD)
+
+# ── Build + start (foreground) ────────────────────────────────────────────────
 up:
 	$(UP)
 

infra/start.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "$0")"
+
+make build
+docker compose up -d