mention how to change internal service port

[gcbw]

This is very useful when running in host mode network.

Is often requested and this is the "official documentation" so far MariaDB/mariadb-docker#334

[tianon]

As the CI's eventually going to point out, this change will need to move from README.md into content.md.

I'm also going to CC @grooverdan and @dbart (the MariaDB image maintainers).

[grooverdan]

While this env obviously does work, it does look a bit out of place compared to other env variables. It also doesn't really have a 3306 default

Any reason we just couldn't recommend --port 3909 or similar in the Connect to MariaDB section which is far too docker generic at the moment? It would prevent reintroducing a MYSQL only naming and hint towards other MariaDB system server command line options that could be used by in the same way.

[gcbw]

This was not intended as an improvement (which i agree with @grooverdan would be to add the explicit default and use --port receiving a proper docker composer variable). This was instead intended as simply documenting the currently accepted work around when using host-only network driver.

[yosifkit]

Yeah, it might make sense to expand the "configuration without a cnf file section" to mention --port in the same way you can customize all the other MariaDB variables, via flags to mariadbd (https://github.com/docker-library/docs/tree/ee77758ebb4ab10e7165c533d887649b55510eb8/mariadb#configuration-without-a-cnf-file).

And the yaml equivalent is command:

version: '3.1'

services:

  db:
    image: mariadb
    environment:
      MYSQL_ROOT_PASSWORD: example
    command: --port 3309