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
```

Dockerfile.dev

@@ -1,27 +1,20 @@
-FROM golang:1.10.1-alpine3.7
+FROM alpine:3.7
 
 LABEL maintainer="Minio Inc <dev@minio.io>"
 
-ENV GOPATH /go
-ENV PATH $PATH:$GOPATH/bin
-ENV CGO_ENABLED 0
+COPY dockerscripts/docker-entrypoint.sh dockerscripts/healthcheck.sh /usr/bin/
+COPY minio /usr/bin/
+
 ENV MINIO_UPDATE off
 ENV MINIO_ACCESS_KEY_FILE=access_key \
     MINIO_SECRET_KEY_FILE=secret_key
 
-WORKDIR /go/src/github.com/minio/
-
-COPY dockerscripts/docker-entrypoint.sh dockerscripts/healthcheck.sh /usr/bin/
-
-COPY . /go/src/github.com/minio/minio
-
 RUN \
      apk add --no-cache ca-certificates 'curl>7.61.0' && \
-     apk add --no-cache --virtual .build-deps git && \
      echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' >> /etc/nsswitch.conf && \
-     cd /go/src/github.com/minio/minio && \
-     go install -v -ldflags "$(go run buildscripts/gen-ldflags.go)" && \
-     rm -rf /go/pkg /go/src /usr/local/go && apk del .build-deps
+     chmod +x /usr/bin/minio  && \
+     chmod +x /usr/bin/docker-entrypoint.sh && \
+     chmod +x /usr/bin/healthcheck.sh
 
 EXPOSE 9000
 

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)