Fix Docker mounted volume performance

See the canonical thread, [File access in mounted volumes extremely slow, CPU bound](https://forums.docker.com/t/file-access-in-mounted-volumes-extremely-slow-cpu-bound/8076), as well as its continuation on GitHub: [File access in mounted volumes extremely slow](https://github.com/docker/for-mac/issues/77). Basically, performance on the Mac (haven't tested other environments) is abysmal for apps that require reading/writing of many or large files. Here are some interim solutions until `osxfs` is up to snuff:
- [docker-sync](http://docker-sync.io/)
- [docker-by-sync](https://github.com/cweagans/docker-bg-sync)
- [docker-sync alternatives](https://github.com/EugenMayer/docker-sync/wiki/Alternatives-to-docker-sync)