Handle --repo in the same way as libostree

[cgwalters]

A major papercut for our CLI right now is

[root@cosa-devsh ~]# ostree container image list
error: The following required arguments were not provided:
    --repo <repo>

USAGE:
    ostree-container container image list --repo <repo>

For more information try --help
[root@cosa-devsh ~]# 

We should automatically use /sysroot/repo in this case, and we should also do the "automatically remount /sysroot rw in a private mountns" dance. This logic currently is private to the ostree CLI in C...we'd need to add some magic here, or perhaps better reimplement in Rust.