Golang：作为makefile的一部分被调用时构建失败

I have the following make file:

default: builddocker

setup:
     go get github.com/golang/protobuf/proto
     go get github.com/golang/protobuf/protoc-gen-go
     go get golang.org/x/net/context
     go get google.golang.org/grpc

buildgo:
    CGO_ENABLED=0 GOOS=linux go build -ldflags "-s" -a -installsuffix cgo -o main ./customer-server/

builddocker:
                    docker build -t johnwesonga/grpc/customer-server -f ./Dockerfile.build .
                    docker run -t johnwesonga/grpc/customer-server /bin/true
                    docker cp `docker ps -q -n=1`:/main .
                    chmod 755 ./main
                    docker build --rm=true --tag=johnwesonga/grpc/customer-server -f Dockerfile.static .

My Dockerfile.build has:

FROM golang

ADD Makefile /
WORKDIR /
RUN make setup

RUN make buildgo
CMD ["/bin/bash"]

But every time I run "make builddocker" I get the following error:

CGO_ENABLED=0 GOOS=linux go build -ldflags "-s" -a -installsuffix cgo -o main ./customer-server/
can't load package: package ./customer-server: open /customer-server: no such file or directory
make: *** [buildgo] Error 1
Makefile:10: recipe for target 'buildgo' failed
The command '/bin/sh -c make buildgo' returned a non-zero code: 2
make: *** [builddocker] Error 2

But if I run "Make buildgo" it works just fine, any idea why this is failing?

FYI my source tree as follows:

/go/src/github.com/johnwesonga/grpc/MakeFile
/go/src/github.com/johnwesonga/grpc/customer-server
/go/src/github.com/johnwesonga/grpc/customer-server/main.go