[Suggestion] Make shell configurable in manifest

[ogarcia]

Is your feature request related to a problem? Please describe.
Not that this is a problem in itself since it is possible to determine the shell using the SHELL environment variable, but it would be interesting if this could be configured in the manifest.

Describe the solution you'd like
In the manifest you can put shell=/bin/bash (for example) and when you run distrobox assemble it uses the indicated shell.

Describe alternatives you've considered
The current alternative is to use the environment variable, but that is not comfortable in a command like assemble. Also, if with assemble you are building two different containers and you want one with one shell and one with a different shell, you can't do it.