ci: bake cached toolchain into Podman agent and fix nested Buildah [CI]

ci/build-runner-image.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Run as the WSL Podman user; retain image layers and tool archives across runs.
+set -euo pipefail
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cache="${MOSHIRO_RUNNER_IMAGE_CACHE:-/home/justme/woodpecker/cache/runner-image}"
+builder_image=localhost/moshiro-dotnet10-nativeaot-aotanywhere:ci
+runner_image=localhost/moshiro-woodpecker-agent:ci
+mkdir -p "$cache/context/ci"
+exec 9> "$cache/.lock"
+flock -w 900 9
+tr -d '\r' < "$root/global.json" > "$cache/context/global.json"
+install -m 0644 "$root/ci/fetch-quic-test-runtime.sh" "$root/ci/woodpecker-agent-entrypoint.sh" "$cache/context/ci/"
+source "$root/ci/nativeaot-toolchain.sh"
+moshiro_nativeaot_settings "$root"
+podman build --layers --pull=missing --isolation chroot --network host \
+  --build-arg "AOTANYWHERE_VERSION=$MOSHIRO_AOTANYWHERE_VERSION" --build-arg "ZIG_TOOLSET_VERSION=$MOSHIRO_ZIG_TOOLSET_VERSION" \
+  -f "$root/packaging/container/NativeAot.Builder.Containerfile" -t "$builder_image" "$cache/context"
+# Export once per image ID. Isolated Buildah stores import this local archive,
+# so parallel architecture builds never download their toolchain from NuGet.
+builder_id="$(podman image inspect --format '{{.Id}}' "$builder_image")"
+builder_id="${builder_id#sha256:}"
+archive="$cache/nativeaot-builder-$builder_id.oci"
+if [[ ! -f "$archive" ]]; then
+  podman save --format oci-archive -o "$archive.tmp" "$builder_image"
+  mv "$archive.tmp" "$archive"
+fi
+printf '%s\n' "$builder_id" > "$cache/nativeaot-builder.id.tmp"
+mv "$cache/nativeaot-builder.id.tmp" "$cache/nativeaot-builder.id"
+printf '%s\n' "$MOSHIRO_NATIVEAOT_BUILDER_KEY" > "$cache/nativeaot-builder.key.tmp"
+mv "$cache/nativeaot-builder.key.tmp" "$cache/nativeaot-builder.key"
+podman build --layers --pull=missing --isolation chroot --network host \
+  --build-arg "NATIVEAOT_BUILDER_IMAGE=$builder_image" \
+  -f "$root/packaging/container/Woodpecker.Agent.Containerfile" -t "$runner_image" "$cache/context"
+echo "built $runner_image; unchanged tool layers are reused"