message: Cache dependencies between tiller-proxy builds
  This brought the build time down from about 30 seconds to under 20
  seconds on my laptop (as long as go.mod and go.sum haven't been
  modified).
diff: FROM golang:1.13 as builder
  -COPY . /go/src/github.com/kubeapps/kubeapps
  WORKDIR /go/src/github.com/kubeapps/kubeapps
  -
  +COPY go.mod go.sum ./
  +RUN go mod download
  +COPY . .
  ARG VERSION
  RUN CGO_ENABLED=0 go build -a -installsuffix cgo -ldflags "-X main.version=$VERSION" ./cmd/tiller-proxy
  