feat(build): borealis-style multi-stage rootfs for the open guest components (#309)

## What

Adds `build/` — a Dockerfile with `mesa-build`, `nestri-build`,
`os-base`, `runtime`, `runtime_prod` and `runtime_debug` stages, plus
the `etc/` overlay, `mkimage.sh` and a `Makefile` — laid out the way
[borealis](https://chromium.googlesource.com/chromiumos/overlays/board-overlays/+/main/project-borealis)
lays out its own `build/`. This moves the guest rootfs formula into this
repo, targeting the four open guest components already here: `nescope`,
`neshub`, `neswire`, `nescapture`.

## Two structural properties worth calling out

- **No privileged host chroot.** A bare `chroot` into a hand-extracted
rootfs needs `/proc`, `/sys`, `/dev` bind-mounted in first. `os-base`
here is `FROM artixlinux/artixlinux:base-openrc` directly with `pacman
-S` as plain `RUN` steps — a Docker build step already has its own
`/proc`/`/sys`/`/dev`.
- **No host-side ownership bug to guard against.** `COPY --from=` runs
as root inside the build with no invoking-user uid in the loop.

## Scope boundary

**Deliberately excludes Proton and Valve's `steamclient.so`** — both
closed, and `CLAUDE.md` forbids closed content in this repo.
`runtime_prod`, tagged `nestrilabs/nestri:base`, is a complete,
bootable, Steam-less image — and also the shared foundation other builds
start from. Whatever layers Proton/Steam on top of it is a closed build
outside this repo, by design.

## Known gap

Nothing starts a payload yet — `nesinit` isn't open code — so
`/etc/init.d/nescope` boots it in plain-compositor mode (no command
after `--`) rather than running a game. Real and testable, just not a
full session yet. Details in `build/README.md`.

## Status

Built and tagged locally as `nestrilabs/nestri:base` (podman, no
`--no-cache` issues, greptile's three findings all fixed and verified
against a real build). Not yet packed into a disk image or run inside
nesbox.

🤖 Generated with [Claude Code](https://claude.com/claude-code)






<!-- greptile_comment -->

<h3>Greptile Summary</h3>

The PR adds a multi-stage Artix/OpenRC guest-rootfs build for the open
Nestri components, with production and debug image flavors.
- Builds patched Mesa and the Rust workspace in dedicated builder
stages.
- Assembles and configures the bootable guest environment and OpenRC
services.
- Packs a selected container image into an ext4 root filesystem while
retaining rootless container storage access.

<h3>Confidence Score: 5/5</h3>

The PR appears safe to merge.

No blocking failure remains.

<h3>Important Files Changed</h3>




| Filename | Overview |
|----------|----------|
| build/Dockerfile | Defines the complete multi-stage build, overlays
repository-root-relative configuration paths, and creates production and
debug runtime targets. |
| build/Makefile | Provides consistent image build and packing targets
using a repository-root context and matching image tags. |
| build/scripts/mkimage.sh | Keeps container-runtime operations in the
invoking user's storage while escalating only filesystem creation and
mounting operations. |
| build/etc/conf.d/nestri-user-env | Supplies the shared service
environment and export function required by the OpenRC service scripts.
|
| build/etc/init.d/guest-net | Configures optional guest networking from
kernel parameters or stable defaults. |
| build/etc/init.d/neswire | Starts the audio sink after its
dependencies and pins it as the PipeWire default once the graph is
ready. |


<h3>Flowchart</h3>

```mermaid
%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Arch builder] --> B[Mesa build]
  A --> C[Nestri workspace build]
  D[Artix OpenRC base] --> E[Common runtime]
  B --> E
  C --> E
  F[build/etc overlay] --> E
  E --> G[runtime_prod]
  E --> H[runtime_debug]
  G --> I[Container image]
  H --> J[Debug container image]
  I --> K[mkimage.sh]
  J --> K
  K --> L[ext4 rootfs]
```

<sub>Reviews (6): Last reviewed commit: ["refactor(build): rename the
published
im..."](https://github.com/nestrilabs/nestri/commit/6fecc8cc31bc425db952193074d4b9b0f861de89)
| [Re-trigger
Greptile](https://app.greptile.com/api/retrigger?id=58981739)</sub>

<!-- /greptile_comment -->

build/etc/conf.d/nestri-user-env

@@ -0,0 +1,30 @@
+HOME="/home/nestri"
+USER="nestri"
+LOGNAME="nestri"
+XDG_RUNTIME_DIR="/run/user/1000"
+XDG_CONFIG_HOME="/home/nestri/.config"
+XDG_DATA_HOME="/home/nestri/.local/share"
+XDG_CACHE_HOME="/home/nestri/.cache"
+XDG_STATE_HOME="/home/nestri/.local/state"
+XDG_DATA_DIRS="/usr/local/share:/usr/share"
+XDG_SESSION_TYPE="wayland"
+XDG_SESSION_DESKTOP="nestri"
+DBUS_SESSION_BUS_ADDRESS="unix:path=/run/user/1000/bus"
+
+# Driver forcing, mirrored from /etc/profile.d/nestri-env.sh. That file only
+# runs for login shells, and nothing here is a login shell — every service
+# below runs from OpenRC. Capture happens through a Vulkan layer, so a
+# process that reached the GPU via native OpenGL would render fine and
+# capture nothing; zink is what makes such a process capturable at all.
+# These are load-bearing. Keep them in step with the profile.d copy.
+#__GLX_VENDOR_LIBRARY_NAME="mesa"
+#MESA_LOADER_DRIVER_OVERRIDE="zink"
+#GALLIUM_DRIVER="zink"
+
+nestri_export_env() {
+    export HOME USER LOGNAME
+    export XDG_RUNTIME_DIR XDG_CONFIG_HOME XDG_DATA_HOME XDG_CACHE_HOME XDG_STATE_HOME
+    export XDG_DATA_DIRS XDG_SESSION_TYPE XDG_SESSION_DESKTOP
+    export DBUS_SESSION_BUS_ADDRESS
+    #export __GLX_VENDOR_LIBRARY_NAME MESA_LOADER_DRIVER_OVERRIDE GALLIUM_DRIVER
+}