I've mentioned this in other places, but my head-canon for Murderbot's appearance has always been white male. Mostly because I figure that the exact kind of morally bankrupt people who would even build SecUnits would use them to fulfill their own power fantasies and so build them to look like themselves - only with more muscles.

And you are correct that there's nothing saying the builders would be white or even male - but in my universe they tend to be, so that's how I think of them.

So they could use Murderbot's gender and appearance to really emphasize that messed up power fantasy aspect - or as you say they could turn it into an action hero and mess up the whole character.