A much simpler solution (for Mac OSX & Docker for Windows) is to replace the host address from localhost to host.docker.internal

DB_HOST=host.docker.internal
DB_DATABASE=databasename
DB_USERNAME=root
DB_PASSWORD=testpassword

Basically the DNS namehost.docker.internal will resolves to the internal IP address used by the host.

NB: If you have changed your address to host.docker.internal but you still receive connection refused error, it’s most probably because MySQL is currently configured to only listen to the local network.

To resolve that, please update the value of the bind_address to 0.0.0.0 in your my.cnf configuration file.

you are trying to connect to mysql in localhost, which is (surprisingly) the reference to the local host. since its a relative address, inside the container it is being resolved as the container own address, and no mysql is awaiting you there... so to solve it - just give it your real host ip instead of localhost or 127.0.0.1.

step 1 - fix .env file:

DB_HOST=<your_host_ip> #run `ifconfig` and look for your ip on `docker0` network
DB_DATABASE=databasename
DB_USERNAME=laravel_server #not root, since we are going to allow this user remote access.
DB_PASSWORD=testpassword

step 2 - create dedicated user:

open your mysql: mysql -u root -p, give your root password, and run the following:

CREATE USER 'laravel_server'@'%' IDENTIFIED BY 'testpassword';
GRANT ALL PRIVILEGES ON databasename.* TO 'laravel_server'@'%'; 
FLUSH PRIVILEGES;

we created the user and gave it the permissions.

step 3 - open mysql to remote access:

we have to make it listening on all interfaces and not just localhost and therefore we run:

sudo sed 's/.*bind-address.*/bind-address=0.0.0.0/' /etc/mysql/mysql.conf.d/mysqld.cnf

(you will be prompted for password. this command is just replacing the line in mysql configuration file)

step 4 - updating:

1. in the project directory: php artisan config:cache
2. service mysql restart

then docker build and run a new container again. it should work for you.

I see two options -

1. Use the private IP of your docker host i.e where mysql server is running.

2. Use the host network mode while running container in case you want to use localhost.

   docker container --net=host ...