feat(ci): use Alpine Docker container for smol musl builds

Added Docker-based Alpine builds for smol binaries to produce native
musl binaries instead of glibc.

Changes:
- Split build step into two: standard platforms and Alpine
- Alpine builds run in alpine:3.19 Docker container
- Uses --platform flag for correct architecture (amd64 for x64, arm64)
- Installs all required build tools in Alpine: nodejs, npm, pnpm,
  python3, make, g++, linux-headers, git, ccache, ninja

Why Docker over QEMU/setup-alpine:
- Faster: Native execution for x64, QEMU only for Alpine container (not compilation)
- Simpler: One command, no complex setup
- Reliable: Official Alpine images, true musl environment
- arm64: Avoids full system emulation during Node.js compilation

Expected build times:
- x64 Alpine: ~50-60 minutes (native speed)
- arm64 Alpine: ~90-120 minutes (minimal QEMU overhead)

This ensures Alpine builds produce proper musl binaries that work
on Alpine Linux and other musl-based systems.

Author: jdalton

.github/workflows/build-smol.yml

@@ -285,10 +285,24 @@ jobs:
         if: (steps.smol-cache-valid.outputs.valid != 'true' || inputs.force) && matrix.platform == 'win32' && steps.ninja-cache-windows.outputs.cache-hit != 'true'
         run: choco install ninja
 
-      - name: Build smol binary
-        if: steps.smol-cache-valid.outputs.valid != 'true' || inputs.force
+      - name: Build smol binary (Standard platforms)
+        if: (steps.smol-cache-valid.outputs.valid != 'true' || inputs.force) && matrix.platform != 'alpine'
         run: pnpm --filter @socketbin/node-smol-builder run build
 
+      - name: Build smol binary (Alpine - Docker)
+        if: (steps.smol-cache-valid.outputs.valid != 'true' || inputs.force) && matrix.platform == 'alpine'
+        run: |
+          docker run --rm \
+            --platform linux/${{ matrix.arch == 'x64' && 'amd64' || 'arm64' }} \
+            -v $PWD:/workspace \
+            -w /workspace \
+            alpine:3.19 \
+            sh -c "
+              apk add --no-cache nodejs npm python3 make g++ linux-headers git ccache ninja && \
+              npm install -g pnpm@10.16.0 && \
+              pnpm --filter @socketbin/node-smol-builder run build
+            "
+
       - name: Verify smol binary
         shell: bash
         run: |