Docker image does not follow changes to `MYSQL_HOST` variable

Initially opened nextcloud/server/issues/40904, was told it's a docker issue, so here I copy over the relevant sections.

Bug description

In the "Auto configuration via environment variables" section of the Docker Readme, one can use MYSQL_HOST to set the database connection info, and NEXTCLUOD_UPDATE=1 to automate installation.

The problem is when we want to change that MySQL_HOST into something else. We change the environment variable, restart the container, and we get this error: SQLSTATE[HY000] [2006] MySQL server has gone away.

We investigate and find that, while the autoconfig.php reads environment variables, the config.php is hard-coded at installation time with the actual IP/PORTS:

<?php
$CONFIG = array (
...
  'dbhost' => '10.66.60.1:9971',
...

OK, let's try to use the CLI command to edit that hard-coded file, like this:

php occ config:system:set dbhost --value 1.2.3.4

We then get this error again: SQLSTATE[HY000] [2006] MySQL server has gone away

So the config:system:set can't fix the dbhost variable without trying to connect to the old, faulty dbhost!

That means the only option is either to manually edit the config file, to overwrite it with sed, or to overwrite it entirely (which sounds complex because it has to keep all the generated secrets already present in it).

The same issue happens with all other environment variables: db pass, user, database name, https override settings, etc.

Steps to reproduce

Trivial Docker-compose example that renames "db" into "db2": https://gist.github.com/gabriel-v/c6e5a1e18686f39649546c1161fadd64

1. Install docker container using auto-configuration environment variable for MYSQL
2. Change the IP/PORT of the MYSQL server
3. Update the environment variables for auto-configuration and restart the Nextcloud server

Expected behavior

Nextcloud starts successfully with the new DB connection info

Installation method

Community Docker image

Nextcloud Server version

confirmed on 26, 27, latest docker images

Work-Around

https://github.com/nextcloud/helm/tree/main/charts/nextcloud#multiple-configphp-file

Create file config/dbhostoverride.config.php with content:

<?php
$CONFIG = array (
  'dbhost' =>  getenv('MYSQL_HOST'),
);

And add keys to this file for each value expected to change (dbport, passwords, etc.)

This solution is the cleanest, but still requires injecting templates into the image for environment variables that we already set.

Suggestions

This causes a great deal of confusion: one expects the environment variable to be the source of truth for configuring the image. Inspecting the autoconfig.php file also does not help, because it's not obvious the $AUTOCONFIG var is used only at install time, while the $CONFIG var is used at run time.

• readme edit suggestion: https://github.com/nextcloud/docker/pull/2085
• maybe the docker image could add a default docker-environment.config.php file with all the envs overridden, like:

$CONFIG = array (
  'dbhost' =>  getenv('MYSQL_HOST') || getenv('POSTGRES_HOST') || ...,
  ... => ...

Start by reading the Docker README's environment-variable configuration section, autoconfig.php, and the generated config.php, then reproduce the db-to-db2 change using the linked Docker Compose example. Compare installation-time values with runtime configuration and review the proposed config/dbhostoverride.config.php approach and pull request 2085. Done means changing the documented environment variables lets the container start with the new database settings without manual config-file edits.