Improve caching

Dockerfile

@@ -1,9 +1,12 @@
-FROM golang:1.11-stretch as builder
+FROM golang:1.12 as deps
 WORKDIR /build
 COPY go.mod go.sum ./
-RUN go get
+RUN go get -mod=readonly
+
+FROM golang:1.12 as builder
+WORKDIR /build
 COPY * ./
-RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o default-request-adder .
+RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -a -installsuffix cgo -o default-request-adder .
 
 FROM scratch
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/