Quickly build dev docker images using 'make docker' (#6505)

This PR simplifies the process of developer build of local
docker containers using `make docker`.

You need to provide a TAG i.e

```
TAG=y4m4/minio:exp make docker
```

Makefile

@@ -75,6 +75,9 @@ build: checks
 	@echo "Building minio binary to './minio'"
 	@CGO_ENABLED=0 go build -tags kqueue --ldflags $(BUILD_LDFLAGS) -o $(PWD)/minio
 
+docker: build
+	@docker build -t $(TAG) . -f Dockerfile.dev
+
 pkg-add:
 	@echo "Adding new package $(PKG)"
 	@${GOPATH}/bin/govendor add $(PKG)