Vendor apple/containerization with a VM-extensions forwarding patch

Switch the containerization dependency from the github URL to a vendored copy
(third_party/containerization, upstream commit 6b7b42ca) referenced by path, so
we can carry a small local patch that upstream lacks: LinuxContainer.Configuration
gains a `vmExtensions` field forwarded into VMConfiguration.extensions. Upstream
already supports VMConfiguration.extensions + the VZInstanceExtension hook, but
LinuxContainer — our only entry point — never forwarded them, so there was no way
to attach a device (e.g. a memory balloon) to a container's VM.

Tests/, docs/, examples/, images/ and the corresponding test targets are trimmed
for footprint (we never build the dependency's tests). See PATCHES.md for the full
diff vs. upstream and the re-vendoring procedure. Also adds the ContainerizationExtras
product to NucleicCore (AddressAllocator, named in the configureVZ signature).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.github/workflows/linux-build.yml

@@ -0,0 +1,60 @@
+name: Linux build
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+  push:
+    branches:
+      - main
+      - release/*
+
+jobs:
+  swift-version:
+    name: Determine Swift version
+    runs-on: ubuntu-24.04
+    outputs:
+      image: ${{ steps.version.outputs.image }}
+    steps:
+      - name: Checkout .swift-version
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8  # v6
+        with:
+          sparse-checkout: .swift-version
+          sparse-checkout-cone-mode: false
+
+      - name: Read Swift version
+        id: version
+        run: echo "image=swift:$(cat .swift-version)-noble" >> "$GITHUB_OUTPUT"
+
+  build:
+    name: Linux compile check
+    needs: swift-version
+    timeout-minutes: 30
+    runs-on: ubuntu-24.04
+    container: ${{ needs.swift-version.outputs.image }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8  # v6
+        with:
+          fetch-depth: 0
+
+      - name: Install system dependencies
+        run: apt-get update && apt-get install -y curl make libarchive-dev libbz2-dev liblzma-dev libssl-dev
+
+      - name: Build containerization
+        run: make containerization
+
+      - name: Build vminitd (glibc)
+        run: make -C vminitd SWIFT_CONFIGURATION="--disable-automatic-resolution -Xswiftc -warnings-as-errors"
+
+      - name: Install Static Linux SDK
+        run: make -C vminitd linux-sdk
+
+      - name: Build vminitd (musl)
+        run: make -C vminitd
+
+      - name: Run unit tests
+        run: swift test --disable-automatic-resolution -Xswiftc -warnings-as-errors