Mac Mini keeps trying to mount encrypted DMG at login
Ok, finally I managed to solve it by myself. Probably there is a faster way to do this, but I didn't find it out, so I went the most obvious way:

Create a new encrypted disk image with Disk Utility, making sure of leaving the "Remember password in keychain" checkbox UNCHECKED (it seems to be checked by default), then I just copied everything from the old encrypted disk image to this new one and finally, I just deleted the old one.

That annoying dialog is not bothering me anymore.

I hope this is useful for someone else.