Mocha tests take 20 seconds to get started in docker?

I'm testing frontend-only-code with mocha, docker-compose and node. The following is run in docker using docker-compose run.

NODE_ENV=test env NODE_PATH=$NODE_PATH:$PWD/src mocha $(find . -name 'test_*.js') --compilers js:babel-core/register -r jsdom-global/register --require babel-polyfill

It takes about 15-20 seconds until the first (!) test starts running.

I thought the docker container would boot fast because after 2 or 3 seconds my shell command (see above) is shown on the console. (Which I assumed, would mean the container is up). But after the command is shown it takes 15 - 20 seconds to run the first test.

What's wrong?

Edit: I'm on a Macbook Pro with macOS