Add ability specify runtime runtime bundle config values when unpacking

[jul-sh]

it would be nice to be able to set runtime config values when unpacking.

The concrete use-case we need for https://github.com/project-oak/oak is the ability to define bind mounts to mount OS dirs into the container.

It'd be nice to be able to pass those as flags to the unpack command, similar to Docker's --mount flag.

[cyphar]

Sorry for not responding earlier. My initial thoughts are that config.json is a fairly straightforward thing to modify with jq and it's quite hard to get something like mount configurations wrong.

But that being said, we do have special handling for VOLUME (and there are some open questions about whether we should have more fully-fledged support for it -- see #130) so maybe adding support for mount configurations wouldn't be too crazy of an idea. Though since it's been more than a year since you opened this issue (and Oak appears to use umoci already) I'm not sure whether you still need this feature...

[cyphar]

Feel free to re-open if you'd like to discuss this further.