Update Go to 1.12.6 and update build flags in Dockerfile (#1092)

Signed-off-by: Vince Prignano <[email protected]>

Dockerfile

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 # Build the manager binary
-FROM golang:1.12.5 as builder
+FROM golang:1.12.6 as builder
 
 ARG ARCH
 
@@ -22,9 +22,13 @@ WORKDIR ${GOPATH}/src/sigs.k8s.io/cluster-api
 COPY pkg/    pkg/
 COPY cmd/    cmd/
 COPY vendor/ vendor/
+COPY go.mod go.mod
+COPY go.sum go.sum
 
 # Build
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=${ARCH} go build -a -ldflags '-extldflags "-static"' -o manager sigs.k8s.io/cluster-api/cmd/manager
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=${ARCH} GO111MODULE=on GOFLAGS="-mod=vendor" \
+    go build -a -ldflags '-extldflags "-static"' \
+    -o manager sigs.k8s.io/cluster-api/cmd/manager
 
 # Copy the controller-manager into a thin image
 FROM gcr.io/distroless/static:latest