Update Makefile targets and doc

Makefile

@@ -2,6 +2,9 @@
 
 BINARY_NAME=middleware
 DOCKER_IMAGE=slack-to-ntfy
+VERSION?=latest
+
+# DOCKER_REGISTRY is optional. If set, it will be used for pushing. Otherwise, images are loaded locally.
 
 # Default target
 help:
@@ -29,7 +32,13 @@ clean:
 	rm -f $(BINARY_NAME)
 
 docker-build:
-	docker build -t $(DOCKER_IMAGE) .
+ifndef DOCKER_REGISTRY
+	# DOCKER_REGISTRY is not set, load locally
+	docker buildx build --platform linux/amd64,linux/arm64 -t $(DOCKER_IMAGE):$(VERSION) --load .
+else
+	# DOCKER_REGISTRY is set, push to registry
+	docker buildx build --platform linux/amd64,linux/arm64 -t $(DOCKER_REGISTRY)/$(DOCKER_IMAGE):$(VERSION) --push .
+endif
 
 docker-run:
 	docker compose up -d

README.md

@@ -8,7 +8,7 @@ A lightweight Go service that acts as a middleware between Slack webhooks and nt
 - ✅ Forwards alerts to self-hosted ntfy servers
 - ✅ Bearer token authentication support
 - ✅ Health check endpoint
-- ✅ Lightweight Docker container (~8MB)
+- ✅ Lightweight Docker container (~8.4MB)
 - ✅ High performance and low resource usage
 
 ## Quick Start
@@ -59,18 +59,18 @@ A lightweight Go service that acts as a middleware between Slack webhooks and nt
 
 ### Build locally
 ```bash
-go build -o middleware main.go
-./middleware
+make build
+make run
 ```
 
 ### Build Docker image
 ```bash
-docker build -t slack-to-ntfy .
+make docker-build
 ```
 
 ### Run tests
 ```bash
-go test ./...
+make test
 ```
 
 ## License