fix Dockerfile builds to work with go modules

Author: deitch

Dockerfile

@@ -6,9 +6,12 @@ WORKDIR /go/src/github.com/packethost/cluster-api-provider-packet
 COPY pkg/    pkg/
 COPY cmd/    cmd/
 COPY vendor/ vendor/
+COPY go.* ./
+COPY tools.go ./
+
 
 # Build
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -a -o manager github.com/packethost/cluster-api-provider-packet/cmd/manager
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -mod=vendor -a -o manager github.com/packethost/cluster-api-provider-packet/cmd/manager
 
 # Copy the controller-manager into a thin image
 FROM alpine:3.10