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/fstab

@@ -0,0 +1,29 @@
+/dev/vda    /          ext4     rw,relatime              0 1
+devtmpfs    /dev       devtmpfs rw,nosuid                0 0
+proc        /proc      proc     rw,nosuid,nodev,noexec   0 0
+sysfs       /sys       sysfs    rw,nosuid,nodev,noexec   0 0
+tmpfs       /tmp       tmpfs    rw,nosuid,nodev,size=64M 0 0
+tmpfs       /run       tmpfs    rw,nosuid,nodev,size=32M,mode=0755 0 0
+tmpfs       /var/log   tmpfs    rw,nosuid,nodev,size=16M 0 0
+# POSIX shared memory. devtmpfs does not provide it and nothing else mounts it,
+# so without this entry /dev/shm does not exist at all and `shm_open` fails.
+#
+# PipeWire itself gets by: it allocates buffers with memfd_create and only
+# falls back to /dev/shm. Kept as a precaution for anything else that uses
+# POSIX shm/semaphores directly — Proton is the known example, and it is not
+# part of this image yet (see build/README.md), but a failure here is silent
+# rather than fatal, so it costs nothing to have ready.
+#
+# `nosuid,nodev` and a size cap because everything a game can write to should
+# have both.
+tmpfs       /dev/shm   tmpfs    rw,nosuid,nodev,size=256M 0 0
+# The guest's logs, on a host directory that outlives the VM.
+#
+# Here rather than mounted by a service, because the failures worth reading
+# are the ones that happen *before* anything else has mounted something — a
+# log directory that appears only after a successful start cannot record an
+# unsuccessful one. The tag is fixed by nessh, so plain fstab works.
+#
+# `nofail` because a VM started without the share still has to boot: that is
+# how somebody gets a shell to find out why it has no share.
+logs        /nestri/logs virtiofs rw,nofail                0 0