Build rtorrent Docker image on Debian 12 slim

- Create multi-stage Dockerfile for libtorrent/rtorrent compilation
- Set up runtime image with binaries, config, and entrypoint script
- Implement Makefile and docker-compose for build/cleanup processes
- Entrypoint script for user creation and config setup

Makefile

@@ -0,0 +1,20 @@
+DOCKER_BUILDKIT ?= 1
+BUILDKIT_PROGRESS ?= auto
+
+DOCKER_COMPOSE_OPTS = BUILDX_GIT_LABELS=full \
+	DOCKER_BUILDKIT=$(DOCKER_BUILDKIT) \
+	BUILDKIT_PROGRESS=$(BUILDKIT_PROGRESS)
+
+DOCKER_COMPOSE_BUILD = $(DOCKER_COMPOSE_OPTS) docker compose -f docker-compose.build.yml build
+
+PRUNE_IMAGES = \
+	localhost/rtorrent:latest
+
+.PHONY: default
+
+default:
+	$(DOCKER_COMPOSE_BUILD) rtorrent
+
+clean:
+	docker image rm $(PRUNE_IMAGES) || true
+	docker builder prune -f