Allow to mount volumes

[agonzalezro]

It shouldn't be that common to use them but sometimes it's useful. For example to mount the token from Kubernetes to be able to access to its API:

+-------------------------------------------------------+
|                                                       |
| kubernetes                                            |
|            +----------------------------------------+ |
|            |                                        | |
|            | botella                                | |
|            |         +----------------------------+ | |
|            |         |                            | | |
|            |         | plugin #1                  | | |
|            |         |           +----------------+ | |
|   /var     |         |           |                | | |
|     +--------->  +---------------->/var/.../token | | |
|     |      |         |           |                | | |
|     |      |         +-----------+----------------+ | |
|     |      |                                        | |
|     |      |         +----------------------------+ | |
|     |      |         |                            | | |
|     |      |         | plugin #2                  | | |
|     |      |         |           +----------------+ | |
|     |      |         |           |                | | |
|     +--------->  +---------------->/var/.../token | | |
|            |         |           |                | | |
|            |         +-----------+----------------+ | |
|            |                                        | |
|            +----------------------------------------+ |
|                                                       |
+-------------------------------------------------------+

PS: the draw was done to clarify, but I am not pretty sure it's doing it :)