RFE: pack, ostree commit, or run commands in a subdirectory

[smcv]

debos knows how to pack up the entire $ROOTDIR into a tarball, but it can't pack up (for example) just /usr into a tarball.

Similarly, it can commit the entire $ROOTDIR to an ostree repository, but can't commit something other than the root. This would be useful for Flatpak runtimes, which are meant to contain:

• /files: a copy of a merged /usr
• /metadata: a metadata file

Workaround: use action: run, chroot: false to run tar or ostree yourself. The way I'm currently doing this is to create ostree/main and ostree/sources in the $ROOTDIR, move usr to ostree/main/files, fetch corresponding source code into ostree/sources, and pack up ostree/sources and ostree/main as though they were my root directory.

If/when I split out time zones and locales into Flatpak extensions, I'll need to commit those separately: for instance, a timezone info extension would contain /files (a copy of /usr/share/zoneinfo) and /metadata.

Similarly, at the moment I'm using a debos recipe for the Platform runtime and a separate debos recipe for the Sdk runtime, and duplicating the common preparation used for both. Ideally my process would go something like this:

• populate $ROOTDIR with a complete Debian system that is slightly larger than the Platform
• do the /usr merge and other common preparation
• copy /usr, /etc and /var twice, for example into /platform and /sdk (hard-link or reflink to avoid duplicating large files)
• chroot into /platform and delete Essential packages that are pointless in an immutable, non-bootable runtime, like login, init, apt and dpkg, then commit/pack
• chroot into /sdk, install extra -dev packages, and download corresponding source code for the union of Platform and Sdk, then commit/pack

Workaround: use chroot: false and make the scripts enter the appropriate chroot themselves, using chroot or systemd-nspawn.