I'm not sure "would" can be used in the way you describe, because it implies a hypothetical, "If [A] happens, then [B] would be true."
That container would hold a gallon (if you put a gallon in it).
I don't think this is more tentative (yes, meaning "expressing uncertainty") than "will". It just implies that there's some hypothetical component to the sentence. Consider these examples:
Donald Trump will not be a great president.
Donald Trump would not be a great president.
Whatever you may believe about the man, at this point in time the second sentence doesn't work because his presidency is no longer hypothetical. It would have been fine to say anytime up to November 9th, with the implied hypothetical "if he becomes president".
Meanwhile, before November 9th, you could have said the first sentence if you really believed he was going to become president -- i.e., that it wasn't a hypothetical. But both are equally certain about him being a bad president.
Of course, you could also say "Donald Trump is a bad president" -- but that's kind of silly since he's not president yet. Ask me again after January 20th, and I might be certain.
There are of course many ways to express uncertainty:
That container might hold a gallon
That container would possibly hold a gallon.
That container should hold a gallon.
Donald Trump will likely be a bad president.
And so on.