Cache "go get" in docker build

I want to encapsulate my golang unit testing in a docker-compose script because it depends on several external services. My application has quite a lot of dependencies, so it takes a while to go get.

How can I cache packages in a way that allows the docker container to build without having to download all dependencies every time I want to test?

My Dockerfile:

FROM golang:1.7

CMD ["go", "test", "-v"]

RUN mkdir -p /go/src/app
WORKDIR /go/src/app

COPY . /go/src/app
RUN go-wrapper download
RUN go-wrapper install

Every time I want to run my unit tests I run docker-compose up --build backend-test on the following script:

version: '2'
services:
  ...
  backend-test:
    build:
      context: .
      dockerfile: Dockerfile
    image: backend-test
    depends_on:
      ...

But now go-wrapper download is called each time I want to run the tests and it takes a looong time to complete.

Solutions? Thanks in advance!