Don't use GOPATH to build

Dockerfile

@@ -1,11 +1,11 @@
 FROM golang:alpine AS builder
 
-ADD . /go
+ADD . /go/src
 
-RUN go build -o sleeper
+RUN cd /go/src; go build -o sleeper
 
 FROM alpine
 
-COPY --from=builder sleeper /sleeper
+COPY --from=builder /go/src/sleeper /sleeper
 
 CMD ["/sleeper"]