Part of the `e2e (stream10)` was running stream9.
As `just` doesn't support named parameters yet,
use `STREAM` env var to select between stream9 and stream10
- Move the Dockerfile to the toplevel as a primary dev entrypoint
- The Justfile is intended especially for agentic AI like
block/goose or Claude Code as an allowlistable-command entrypoint
- Include attempt at incremental build caching, partially defeated
by autotools
- Add new tests-unit-container that tests ostree-prepare-root in
a container
Signed-off-by: Colin Walters <walters@verbum.org>
