  Recent numerical relativity simulations within the
Einstein--Maxwell--(charged-)Klein-Gordon (EMcKG) system have shown that the
non-linear evolution of a superradiantly unstable Reissner-Nordstr\"om black
hole (BH) enclosed in a cavity, leads to the formation of a BH with scalar
hair. Perturbative evidence for the stability of such hairy BHs has been
independently established, confirming they are the true endpoints of the
superradiant instability. The same EMcKG system admits also charged scalar
soliton-type solutions, which can be either stable or unstable. Using numerical
relativity techniques, we provide evidence that the time evolution of some of
these $\textit{unstable}$ solitons leads, again, to the formation of a hairy
BH. In some other cases, unstable solitons evolve into a (bald)
Reissner-Nordstr\"om BH. These results establish that the system admits two
distinct channels to form hairy BHs at the threshold of superradiance: growing
hair from an unstable (bald) BH, or growing a horizon from an unstable
(horizonless) soliton. Some parallelism with the case of asymptotically flat
boson stars and Kerr BHs with scalar hair is drawn.
