Let $X$ be a compact orientable non-Haken 3-manifold modeled on the Thurston
geometry $\text{Nil}$. We show that the diffeomorphism group $\text{Diff}(X)$
deformation retracts to the isometry group $\text{Isom}(X)$. Combining this
with earlier work by many authors, this completes the determination the
homotopy type of $\text{Diff}(X)$ for any compact, orientable, prime 3-manifold
$X$.