Support writable filesystems

[multimeric]

I realise you've already started implementing ext4 support, but for my shifter use-case I'd love for my image to be able to modify its own filesystem.

So just a +1 for implementing this

[dmjacobsen]

Hi Miguel,

We're starting work on a new feature using overlayfs that may allow some of what you're looking for. The ext4 stuff would still need to be mounted readonly if used in parallel, while overlay will allow in-memory manipulations of the container --- assuming the container had sufficiently permissive permissions on the specific paths that one wanted to modify to allow it (user's may not act as root in shifter).

-Doug