feat: Expand platform support to match Alpine's supported architectures

- Add support for linux/386 (i686-unknown-linux-musl)
- Add support for linux/arm/v6 (arm-unknown-linux-musleabihf)
- Add support for linux/ppc64le (powerpc64le-unknown-linux-musl)
- Add support for linux/s390x (s390x-unknown-linux-musl)
- Add support for linux/riscv64 (riscv64gc-unknown-linux-musl)
- Update platform detection to include all new platforms
- Add tests for all new platform mappings
- Update documentation with installation instructions for all targets
- Add CI job to test extended platform support
- Update development setup docs to mention cargo-zigbuild for full platform support

This allows krust to build images for all platforms supported by Alpine Linux,
making it more versatile for multi-architecture container deployments.

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

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -166,6 +166,67 @@ jobs:
         # Run the image - should automatically select the right architecture
         docker run --rm $IMAGE_REF
 
+  # Test extended platform support
+  extended-platforms:
+    name: Extended Platform Support
+    runs-on: ubuntu-latest
+    needs: [test]  # Only run after basic tests pass
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        targets: x86_64-unknown-linux-musl,i686-unknown-linux-musl,aarch64-unknown-linux-musl,armv7-unknown-linux-musleabihf
+    - name: Install cross-compilation tools
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y musl-tools gcc-multilib
+    - name: Setup cargo config
+      run: |
+        mkdir -p .cargo
+        cat > .cargo/config.toml << 'EOF'
+        [target.x86_64-unknown-linux-musl]
+        linker = "x86_64-linux-musl-gcc"
+
+        [target.i686-unknown-linux-musl]
+        linker = "musl-gcc"
+        rustflags = ["-C", "target-cpu=i686"]
+
+        [target.aarch64-unknown-linux-musl]
+        linker = "aarch64-linux-gnu-gcc"
+
+        [target.armv7-unknown-linux-musleabihf]
+        linker = "arm-linux-gnueabihf-gcc"
+        EOF
+    - name: Build krust
+      run: cargo build --release
+    - name: Test multi-platform build with Alpine base
+      run: |
+        # Create a test project that uses Alpine (which supports many platforms)
+        mkdir -p test-alpine-platforms/src
+        cat > test-alpine-platforms/Cargo.toml << 'EOF'
+        [package]
+        name = "test-alpine"
+        version = "0.1.0"
+        edition = "2021"
+
+        [package.metadata.krust]
+        base-image = "alpine:latest"
+        EOF
+        cat > test-alpine-platforms/src/main.rs << 'EOF'
+        fn main() {
+            println!("Hello from Alpine multi-platform test!");
+        }
+        EOF
+
+        # Test that platform detection works (even if we can't build all platforms)
+        cd test-alpine-platforms
+        ../target/release/krust build --no-push --image test.local/alpine-test:latest . 2>&1 | tee build.log
+
+        # Verify platform detection happened
+        grep -q "Detecting available platforms from base image: alpine:latest" build.log
+        grep -q "Found platforms:" build.log
+
   # Takes too long to run on CI, so it's commented out for now.
   # coverage:
   #   name: Code coverage