feat: add Docker deployment support

Add Dockerfile, docker-compose.yml, .dockerignore, .env.example, Makefile docker targets, and README/CHANGELOG docs.

Makefile

@@ -1,7 +1,7 @@
 BINARY := yDropbox
 PKG := ./cmd/ydropbox
 
-.PHONY: build test run clean vet
+.PHONY: build test run clean vet docker docker-up docker-down
 
 build:
 	CGO_ENABLED=0 go build -o $(BINARY) $(PKG)
@@ -17,3 +17,12 @@ vet:
 
 clean:
 	rm -f $(BINARY)
+
+docker:
+	docker build -t ydropbox .
+
+docker-up:
+	docker compose up -d --build
+
+docker-down:
+	docker compose down