How to refer from one container to another using localhost - docker

Question

I'm working on building docker containers for a Ruby-on-Rails project I'm currently working on, so that I can develop this project using the remote feature of Visual Studio Code. This project should still continue to work without using docker containers, so I cannot make breaking changes to the existing code that would compromise this.
The application server (Rails) needs to connect to a MySQL database that's running in a separate container. The database container is named db, and I can connect from the application container to this container by using the db hostname.
The database.yml config file for rails defines how to connect to the database, but this is where my problem is situated. I don't want to change the host to db instead of localhost as this would mean that regular users (that do not use Docker containers) will no longer be able to connect to the database without changing this file. How can I somehow start or change my docker config so that db is accessible as localhost instead of db inside of the application container?
database.yml:
default: &default
adapter: mysql2
username: ****
password: ****
pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
development:
<<: *default
database: ****
# setup local port forwarding for this to work
host: db
port: 3306
docker-compose.yml:
version: '3.7'
services:
db:
build: ./unipept-db
environment:
MYSQL_ROOT_PASSWORD: ****
MYSQL_DATABASE: ****
MYSQL_USER: ****
MYSQL_PASSWORD: ****
restart: always
ports:
- "3306:3306"
hostname: mysql
phpmyadmin:
depends_on:
- db
image: phpmyadmin/phpmyadmin
ports:
- '8080:80'
environment:
PMA_HOST: db
MYSQL_ROOT_PASSWORD: ****
restart: always
app:
depends_on:
- db
build: ./unipept-application
command: sleep infinity
ports:
- '5000:5000'
volumes:
- ~/.gitconfig:/root/.gitconfig
- ..:/workspace

Answer 1

user network_mode: "host"in your APP config then you can call the DBfrom your APP using localhost:PORT
phpmyadmin:
depends_on:
- db
image: phpmyadmin/phpmyadmin
ports:
- '8080:80'
environment:
PMA_HOST: db
MYSQL_ROOT_PASSWORD: ****
restart: always
network_mode: "host"
app:
depends_on:
- db
build: ./unipept-application
command: sleep infinity
ports:
- '5000:5000'
network_mode: "host"
volumes:
- ~/.gitconfig:/root/.gitconfig
- ..:/workspace
PS: Published ports are discarded when using host network mode

Answer 2

If you make it an environment variable in your database.yml.erb file, it will be configurable. You even already have an example of this. You can set:
host: <%= ENV.fetch('DB_HOST', 'localhost') %>
In development, just don't set the environment variable, and it will use localhost. In a Docker environment, do set it, and it will use that hostname.
version: '3'
services:
db:
image: mysql
app:
build: .
environment:
DB_HOST: db
ports:
- '5000:5000'
You should also pass things like database credentials the same way.