Build from local KamadoPool checkout via buildx build-context

Replaces the git-clone-at-SHA source stage with a 'FROM scratch
AS source' stage that COPYs from a named build context. The
Makefile passes '--build-context kamado=../KamadoPool' by
default; override with 'make KAMADO_SRC=/elsewhere'. No more
SHA placeholder, no GitHub dependency, no re-clone on every
build — and the image always reflects the working tree.

Makefile

@@ -19,10 +19,13 @@ $(PKG_ID).s9pk: manifest.yaml instructions.md icon.png LICENSE scripts/embassy.j
 scripts/embassy.js: $(TS_FILES)
 	deno run --allow-read --allow-write --allow-env --allow-net scripts/bundle.ts
 
+KAMADO_SRC ?= ../KamadoPool
+
 image.tar: Dockerfile docker_entrypoint.sh
 	docker buildx build --tag start9/$(PKG_ID)/main:$(PKG_VERSION) \
 		--platform=linux/arm64/v8,linux/amd64 \
 		--build-arg ARCH=$(shell uname -m) \
+		--build-context kamado=$(KAMADO_SRC) \
 		-o type=oci,dest=image.tar .
 
 clean: