Add GOOS buildarg in dockerfile

Signed-off-by: Victor Vieux <[email protected]>

Dockerfile

@@ -1,5 +1,7 @@
 FROM golang:1.5.3-alpine
 
+ARG GOOS
+
 COPY . /go/src/github.com/docker/swarm
 WORKDIR /go/src/github.com/docker/swarm
 
@@ -8,7 +10,7 @@ ENV GOPATH /go/src/github.com/docker/swarm/Godeps/_workspace:$GOPATH
 RUN set -ex \
 	&& apk add --no-cache --virtual .build-deps \
 	git \
-	&& CGO_ENABLED=0 go install -v -a -tags netgo -installsuffix netgo -ldflags "-w -X github.com/docker/swarm/version.GITCOMMIT `git rev-parse --short HEAD` -X github.com/docker/swarm/version.BUILDTIME \"`date -u`\""  \
+	&& GOOS=$GOOS CGO_ENABLED=0 go install -v -a -tags netgo -installsuffix netgo -ldflags "-w -X github.com/docker/swarm/version.GITCOMMIT `git rev-parse --short HEAD` -X github.com/docker/swarm/version.BUILDTIME \"`date -u`\""  \
 	&& apk del .build-deps
 
 ENV SWARM_HOST :2375