update Dockerfile to use go.mod

Author: Jefftree

artifacts/images/agent-build.Dockerfile

@@ -3,6 +3,15 @@ FROM golang:1.12.1 as builder
 
 # Copy in the go src
 WORKDIR /go/src/sigs.k8s.io/apiserver-network-proxy
+
+# Copy the Go Modules manifests
+COPY go.mod go.mod
+COPY go.sum go.sum
+
+# Cache dependencies
+RUN go mod download
+
+# Copy the sources
 COPY pkg/    pkg/
 COPY cmd/    cmd/
 COPY proto/  proto/

artifacts/images/client-build.Dockerfile

@@ -3,6 +3,15 @@ FROM golang:1.12.1 as builder
 
 # Copy in the go src
 WORKDIR /go/src/sigs.k8s.io/apiserver-network-proxy
+
+# Copy the Go Modules manifests
+COPY go.mod go.mod
+COPY go.sum go.sum
+
+# Cache dependencies
+RUN go mod download
+
+# Copy the sources
 COPY pkg/    pkg/
 COPY cmd/    cmd/
 COPY proto/  proto/

artifacts/images/server-build.Dockerfile

@@ -3,6 +3,15 @@ FROM golang:1.12.1 as builder
 
 # Copy in the go src
 WORKDIR /go/src/sigs.k8s.io/apiserver-network-proxy
+
+# Copy the Go Modules manifests
+COPY go.mod go.mod
+COPY go.sum go.sum
+
+# Cache dependencies
+RUN go mod download
+
+# Copy the sources
 COPY pkg/    pkg/
 COPY cmd/    cmd/
 COPY proto/  proto/