Symptoms are very similar to what I was seeing - when I looked at my log output (from the openeats app docker container) - I could see for some reasons, that the the application (I labeled mine openeats-app) was trying to connect to the openeats-app as the database server. (I have created a seperate mariadb docker container called openeats-mariadb).
I assume somewhere along the way of environment variables, templates, shell scripts etc. the wrong hostname was being picked up / replaced along the way.
I spent a little time trying to troubleshoot - to the point that within the container I was running the schema commands from /code/base/migrate.py pythong file and was getting the same errors - database connection errors. Which make sense given it was trying to connect to the app container. (for the record, after having shelled into the app container, the command I was running as an example was "python /code/base/migrate.py reset_db")
In the end I gave up and just ran the container with MYSQL_USER set to root - as soon as I did that, the app ran as expected.
Happy to share more info, but thought I would include the above in case it helps anyone get started, or for anyone wanting to do more in-depth troubleshooting.
(My gut feel is that the errors is in environment variables - the out put from the shell 'set' command differed from the output of 'env' - mainly in differences between / missing database related vars - I just ran out of time/patience trying to track this down)
(As always, thank you to the maintainer for putting the work together for the rest of us to use)