The documentation on DockerHub states this:

Configure via configuration file

Custom configuration can be given in an additional configuration file (e.g. my-maxscale.cnf). The file needs to be mounted into /etc/maxscale.cnf.d/

docker run -d --name mxs -v $PWD/my-maxscale.cnf:/etc/maxscale.cnf.d/my-maxscale.cnf mariadb/maxscale:latest

This has two problems:

• The [maxscale] section can only be located in the root configuration file /etc/maxscale.cnf. The current documentation does not take this into account and if global options are specified, the startup will fail with an error.
• Mounting the configuration file on top of the root configuration file will overwrite the threads value which can result in a OOM situation if the container has resource restrictions which is why the threads=1 is set by default in the default configuration file.