How can I persist go 1.11 modules in a Docker container?

I am migrating a Go 1.10 app to Go 1.11. This also includes migrating from dep to mod for managing dependencies.

As the application depends on a database, I am using a docker-compose to set up the local development environment. With Go 1.10 I simply mounted the local repository (including the vendor folder) into the correct location in the container's GOPATH:

web:
  image: golang:1.10
  working_dir: /go/src/github.com/me/my-project
  volumes:
    - .:/go/src/github.com/me/my-project
  environment:
    - GOPATH=/go
    - PORT=9999
  command: go run cmd/my-project/main.go

Since Go 1.11 ditches GOPATH (when using modules that is) I thought I could just do the following:

web:
  image: golang:1.11rc2
  working_dir: /app
  volumes:
    - .:/app
  environment:
    - PORT=9999
  command: go run cmd/my-project/main.go

This works, but every time I docker-compose up (or any other command that calls through to the Go tool) it will resolve and re-download the dependency tree from scratch. This does not happen (rather only once) when I run the command outside of the container (i.e. on my local OS).

How can I improve the setup so that the Docker container persists the modules being downloaded by the go tool?

Solution

This is not mentioned in the wiki article on modules, but from reading the updated docs on the go tool, I found out that when using Go modules, the go tool will still use GOPATH to store the available sources, namely $GOPATH/pkg/mod.

This means that for my local dev setup, I can 1. define the GOPATH in the container and 2. mount the local $GOPATH/pkg/mod into the container's GOPATH.

web:
  image: golang:1.11rc2
  working_dir: /app
  volumes:
    - .:/app
    - $GOPATH/pkg/mod:/go/pkg/mod
  environment:
    - GOPATH=/go
    - PORT=9999
  command: go run cmd/my-project/main.go