Compiling plugins with external libraries is painfully slow on macOS

[LukeShu]

Because we use --volume to expose the host's Go module cache to the Docker build image, and because Docker for Mac's osxfs really does not perform well for that use-case.

The first workaround I would try would be to use a Dockerfile to copy the module cache in as a tarball, so that it's on a native Linux filesystem once we get around to actually compiling things. Similar to the trick used in teleproxy.git's Makefile. Maybe even only copy the modules listed in go list -m all, to avoid copying the entire cache.