feat: optional MySQL/MariaDB database driver for deployments that cannot use local SQLite

docs/docker.md

@@ -62,7 +62,7 @@ The container receives SIGTERM on stop, and RenewCMS shuts down gracefully withi
 
 ## Data volume
 
-State lives on the named volume `renewcms-data` (Compose prefixes it with the project name, usually `renewcms_renewcms-data`; check with `docker volume ls`). It holds the SQLite database, uploads, and update records.
+State lives on the named volume `renewcms-data` (Compose prefixes it with the project name, usually `renewcms_renewcms-data`; check with `docker volume ls`). It holds the SQLite database (unless you use MySQL), uploads, and update records.
 
 - `docker compose down` removes the container and keeps the volume. The data survives and `docker compose up -d` brings it back.
 - **`docker compose down -v` deletes the volume and all data.** The next start is an empty instance.
@@ -86,6 +86,22 @@ docker run --rm -v VOLUME:/data -v "$PWD":/backup alpine sh -c "tar xzf /backup/
 docker compose up -d
 ```
 
+## MySQL or MariaDB
+
+SQLite on the `/data` volume is the default. [`docker-compose.mysql.yml`](../docker-compose.mysql.yml) runs RenewCMS next to a MySQL 8.4 container instead:
+
+```sh
+export MYSQL_ROOT_PASSWORD=<generated>
+export DB_PASSWORD=<generated>
+docker compose -f docker-compose.mysql.yml up -d
+```
+
+It starts the `mysql` service with a named volume `mysql-data` and a healthcheck, and starts `renewcms` only once the database is healthy (`depends_on` with `service_healthy`). RenewCMS gets `DB_DRIVER=mysql`, `DB_HOST=mysql`, `DB_NAME`, `DB_USER` and `DB_PASSWORD` from the compose file and the rest from `docker.env`. `DATA_DIR=/data` keeps update state and the `.sql` pre-update backups on the `renewcms-data` volume, which still holds the uploads. Replace the placeholders; never commit real passwords.
+
+To use MariaDB, change the `mysql` service image to `mariadb:11`, its `MYSQL_*` variables to `MARIADB_*` and its healthcheck to `healthcheck.sh --connect --innodb_initialized`. To use a server outside Compose, drop the `mysql` service and point `DB_HOST` at it. For a server on another machine set `DB_TLS=true`.
+
+Back the database up with `docker compose -f docker-compose.mysql.yml exec mysql mysqldump ...` (a `tar` copy of the volume is not consistent while it runs), and the `renewcms-data` volume as described above. Moving an existing SQLite volume is a manual procedure, see [deployment.md](deployment.md#moving-an-existing-sqlite-site-to-mysql).
+
 ## Reverse proxy and TLS
 
 With `ENVIRONMENT=production` the site must be served over HTTPS, so put a reverse proxy in front and set `PUBLIC_URL` to the public `https://` address. Bind the published port to the host only when the proxy runs on the same host, by changing `ports` to `"127.0.0.1:8080:8080"`.