The Drunken Lizard Pub

Is there a key somewhere for the door where bonedust is in the chest?

If we're talking about the same door, then no. Not until you download and install MyGameCompany's mod.
But, in the vanilla game, there is no way into Rockhammer Mine Lv. 3 or further.

I described the door poorly I think. We're probably talking about the same door, but I'm confused at your mention of level 3. This is the one I mean:

[+] SPOILER

Mine2.jpg (134.74 KiB) Viewed 5709 times

Nope, there doesn't appear to be a key for that door. You have to pick the lock, use the Lock Melt spell, or bash it down.

Or now you have a use for all those powder kegs.

Okay. So we were not talking about the same door, but it looks like your problem has been solved.