To have cross-domain access to my test instance of SQL Server, I need a login with a database user, not a Windows user.

As soon as this works, I could start doing it from my * NIX environment, but basically Windows will not allow me to access the database in this way and it will generate a validation error 28000 / SQL Server Error 18456.
I already did
ALTER LOGIN [administrator] WITH DEFAULT_DATABASE=[...]
ALTER LOGIN [administrator] WITH PASSWORD=...
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