Re implement branch based

Dockerfile

@@ -1,13 +1,13 @@
 FROM golang:1.9.2-alpine3.6 AS build
-
+ARG BRANCH=master
 RUN apk add --no-cache make git
 
 RUN go get -u github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway
 RUN go get -u github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger
 RUN go get -u github.com/golang/protobuf/protoc-gen-go
-RUN go get -u github.com/spacemeshos/go-spacemesh
 RUN go get -u github.com/kardianos/govendor
-RUN cd src/github.com/spacemeshos/go-spacemesh; govendor sync; make
+RUN echo ${BRANCH}
+RUN mkdir -p src/github.com/spacemeshos; cd src/github.com/spacemeshos; git clone https://github.com/spacemeshos/go-spacemesh; cd go-spacemesh; git checkout ${BRANCH}; go build; govendor sync; make
 RUN cp /go/src/github.com/spacemeshos/go-spacemesh/config.toml /go
 
 ENTRYPOINT /go/src/github.com/spacemeshos/go-spacemesh/go-spacemesh