feat: add Docker image build target and deployment docs

Makefile

@@ -1,15 +1,20 @@
 APP_NAME = ddns-updater
-BIN_DIR = bin
-VERSION = 1.0.0
+BIN_DIR  = bin
+VERSION  = 1.0.0
 
 # Go parameters
-GOCMD = go
+GOCMD   = go
 GOBUILD = $(GOCMD) build
 GOCLEAN = $(GOCMD) clean
-GORUN = $(GOCMD) run
+GORUN   = $(GOCMD) run
+
+# Docker image (override: make build-dockerimg IMAGE=kccleoc/ddns-updater:latest)
+IMAGE ?= kccleoc/ddns-updater:latest
 
 # Build targets
-.PHONY: all build clean run init-config install install-linux install-macos service-linux service-macos
+.PHONY: all build build-all clean run init-config install \
+        install-linux install-macos service-linux service-macos \
+        build-dockerimg push-dockerimg
 
 all: build
 
@@ -22,8 +27,8 @@ build:
 # Build for multiple platforms
 build-all:
 	mkdir -p $(BIN_DIR)
-	GOOS=linux GOARCH=amd64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-linux-amd64 main.go
-	GOOS=linux GOARCH=arm64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-linux-arm64 main.go
+	GOOS=linux  GOARCH=amd64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-linux-amd64  main.go
+	GOOS=linux  GOARCH=arm64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-linux-arm64  main.go
 	GOOS=darwin GOARCH=amd64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-darwin-amd64 main.go
 	GOOS=darwin GOARCH=arm64 $(GOBUILD) -o $(BIN_DIR)/$(APP_NAME)-darwin-arm64 main.go
 	@echo "Built all platform binaries in $(BIN_DIR)/"
@@ -90,3 +95,13 @@ service-macos:
 	@echo "  launchctl start com.ddns-updater"
 	@echo "\nView logs with:"
 	@echo "  tail -f /usr/local/var/log/ddns-updater.log"
+
+# Build Docker image (parameterized)
+# Usage: make build-dockerimg IMAGE=kccleoc/ddns-updater:latest
+build-dockerimg:
+	docker build -t $(IMAGE) .
+
+# Push Docker image
+# Usage: make push-dockerimg IMAGE=kccleoc/ddns-updater:latest
+push-dockerimg:
+	docker push $(IMAGE)