docs: add dev containers config gotcha in README

[nshki]

Overview

This is a small README update for devs who choose to use dev containers to work on the project. When setting this up on my machine (Fedora Asahi Linux running on an M2 MacBook Air) via the dev containers CLI, I ran into an issue where the Rails server kept throwing a could not connect to server: Cannot assign requested address error. The issue was that the .env file that I copied from .env.example was trying to connect to DB_HOST=localhost when it needed to be pointed at DB_HOST=db in the Docker Compose setup.

This adds a sentence in the README to address this and it also adds a comment in .env.example next to the relevant line.

And as a side note, I'm very excited that you all have decided to open source this lovely project. I'm looking forward to keeping up with its developments and would love to contribute when I can.