Extend Dockerfile to use plain multi stage build

Dockerfile

@@ -1,3 +1,14 @@
+FROM golang:1.10.2 as builder
+
+WORKDIR /go/src/github.com/keycloak/keycloak-gatekeeper
+COPY . ./
+
+RUN go get -u github.com/golang/dep/cmd/dep \
+    && dep ensure \
+    && go test
+
+RUN CGO_ENABLED=0 go build -a -ldflags '-s' -installsuffix cgo -o bin/keycloak-gatekeeper .
+
 FROM alpine:3.7
 
 LABEL Name=keycloak-gatekeeper \
@@ -8,8 +19,9 @@ LABEL Name=keycloak-gatekeeper \
 RUN apk add --no-cache ca-certificates
 
 ADD templates/ /opt/templates
-ADD bin/keycloak-gatekeeper /opt/keycloak-gatekeeper
+
+COPY --from=builder /go/src/github.com/keycloak/keycloak-gatekeeper/bin/keycloak-gatekeeper /opt/keycloak-gatekeeper
 
 WORKDIR "/opt"
 
-ENTRYPOINT [ "/opt/keycloak-gatekeeper" ]
+ENTRYPOINT ["/opt/keycloak-gatekeeper"]