feat: support Alpine Linux containers (#22)

## Summary
- Fall back to `sh` when `bash` is not available, enabling Alpine Linux
support without requiring bash as a prerequisite
- Add "Alpine Container" usage example to README
- Add `test-alpine-container` CI job testing both `latest` and `alpha`
versions in Alpine 3.21 container

## Test plan
- [ ] `test-alpine-container` CI job passes for both vp versions
- [ ] `vp --version` and `vp exec` work inside Alpine container
- [ ] Existing tests on ubuntu/macos/windows still pass

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

---------

Signed-off-by: MK (fengmk2) <[email protected]>

Author: fengmk2

.github/workflows/test.yml

@@ -231,6 +231,33 @@ jobs:
         run: |
           echo "NODE_AUTH_TOKEN is set: ${NODE_AUTH_TOKEN:+yes}"
 
+  test-alpine-container:
+    strategy:
+      fail-fast: false
+      matrix:
+        version: [latest, alpha]
+    runs-on: ubuntu-latest
+    container:
+      image: alpine:3.21
+    steps:
+      - name: Install Alpine dependencies
+        run: apk add --no-cache bash curl gcompat libstdc++
+
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
+
+      - name: Setup Vite+ (${{ matrix.version }})
+        uses: ./
+        with:
+          version: ${{ matrix.version }}
+          run-install: false
+          cache: false
+
+      - name: Verify installation
+        run: vp --version
+
+      - name: Verify vp exec works
+        run: vp exec node -e "console.log('vp exec works in Alpine')"
+
   build:
     runs-on: ubuntu-latest
     steps:

README.md

@@ -110,6 +110,22 @@ steps:
       NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
 ```
 
+### Alpine Container
+
+Alpine Linux uses musl libc instead of glibc. Install compatibility packages before using the action:
+
+```yaml
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: alpine:3.21
+    steps:
+      - run: apk add --no-cache bash curl gcompat libstdc++
+      - uses: actions/checkout@v6
+      - uses: voidzero-dev/setup-vp@v1
+```
+
 ### Matrix Testing with Multiple Node.js Versions
 
 ```yaml