feat: support add groups for user

[XiYang6666]

[crazy-max]

Can you also add a new user with multiple groups in this file https://github.com/crazy-max/docker-samba/blob/master/test/data/config.yml so it's tested in ci workflow?

[XiYang6666]

auth:
  - user: foo
    group: foo
    uid: 1000
    gid: 1000
    password: bar
    groups: [qux, quux]
  - user: yyy
    group: xxx
    uid: 1100
    gid: 1200
    password_file: /tmp/yyy_password

group:
  qux: 2001
  quux: 2002

I think this is a better implementation because each user requires a main group. Additionally, this implementation can also ensure that groups are not created repeatedly.

[spaghetti-coder]

Nice one! You spared me a MR with a more clumsy implementation Think will also be good to see the usage in examples here: https://github.com/crazy-max/docker-samba/tree/master/examples The entries added to readme do nothing useful in terms of samba shares from the same conf file