Regarding construction, the cost is always exactly the same (unlike repairs), no matter what you do with or without engineers. Collic is right.
In terms of whether or not it is ideal to have more engineers on one dock, or multiple docks, that's been debated in the past but the math has changed a lot now that both docks and engineers costs so much less energy (energy was the big question mark in the past, usually, which led to more engineers on one dock being the ideal in the past).
When you have a bunch of engineers on one dock, there's a big limiting factor there: ships can't be produced faster than about 1-2 per second, max. So if a ship already takes 3 seconds normally, and then you put two engineers on that, your incremental benefit from putting on more engineers is extremely marginal after a certain point. With more time-consuming ships, like starships for example, of course it's way longer before the marginal benefit falls; it would take an insane number of engineers to make those produce at more than 1 per second, and I don't think any economy could handle it.
But with fleet ships it's common, so having multiple docks is probably ideal in those situations for production speed -- though others may disagree based on energy efficiency.