• One-line dev environment setup and launch: docker-compose up.
• One easy-to-install dependency to get coding on a new computer: Docker. (Versus two complex ones when using Vagrant.)
• Development-oriented config: Source code is mounted so that changes in the container appear on the host, and vice-versa.
• Fast re-builds because the DOCKERFILE is written to help Docker cache the images.
• Syncing with Postgres startup delay.
• All the crappy little dependencies installed.
• No weird hacks.
• Tested with current Ruby and Rails as of 2018-02-02: Ruby 2.5.0, Rails 5.1.4

Tested on MacOS and Fedora Linux, because that's what I happen to use. This is my personal configuration that I use for client work and my own projects. It's the result of several late-night hours getting everything working!

0. Install Docker somehow. I use Docker for Mac.
1. Copy the three config files to an existing Rails project. Make run.sh executable, e.g., chmod +x run.sh.
2. Edit your development database settings to connect to Postgres at host db, username postgres, password empty string.
3. Spin it up with docker-compose up.

Tests can be run in the container like so:

docker-compose exec web bin/rspec

Or use Spring inside Docker the cheap and easy way:

docker-compose exec web bash
bin/rspec

...then leave that session open and re-run the tests with a fast Spring boot up:

bin/rspec