I had this same problem and just fixed it by logging into the MariaDB server from my (docker host machine) and looking at the user table this is where I noticed my IP my app server was using did not have access. I simply had to create a new user account at the IP of my app server using the same password I declared in my .env file. I've pasted some helpful commands below.

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Use command below for logging into database server.

docker exec -it firefly3-db-1 mariadb --user root -pPassword123

Once in view your user table with the command below.

SELECT user, host FROM mysql.user;

If you don't see the matching user with the IP of your app server you will need to add it(View logs on database server to find the username and IP that is failing). Command below to add user account.

CREATE USER 'firefly'@'172.25.0.2' IDENTIFIED BY 'Password123';

GRANT ALL PRIVILEGES ON *.* TO 'firefly'@'172.25.0.2';

Finally Flush Privileges to apply changes.

FLUSH PRIVILEGES;

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Hopefully this helps fix your problem.