unpack: cannot handle non-numeric user/groups

[cyphar]

When generating the config.json, we can't handle cases where the user specification is something like "cyphar:users" because of the way the interfaces are designed (we do the extraction and generation separately from one another). This can be fixed by using libcontainer/users.

In addition, we'll also be able to fill the AdditionalGids array, which we couldn't before.

We could also add a $HOME env variable.

[cyphar]

This is fixed in 12eacec.