Dockerfile: optimize incremental build a bit, copy all binaries to output

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

Dockerfile

@@ -5,11 +5,15 @@
 FROM golang:1.13-alpine AS build-env
 
 WORKDIR /go/src/tailscale
+
+COPY go.mod .
+COPY go.sum .
+RUN go mod download
+
 COPY . .
 
-RUN go get -d -v ./cmd/...
 RUN go install -v ./cmd/...
 
 FROM alpine:3.11
 RUN apk add --no-cache ca-certificates
-COPY --from=build-env /go/bin/tail* /usr/local/bin/
+COPY --from=build-env /go/bin/* /usr/local/bin/