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>

README.md

@@ -42,6 +42,32 @@ rustup target add aarch64-unknown-linux-musl
 
 # For linux/arm/v7
 rustup target add armv7-unknown-linux-musleabihf
+
+# For linux/arm/v6
+rustup target add arm-unknown-linux-musleabihf
+
+# For linux/386
+rustup target add i686-unknown-linux-musl
+
+# For linux/ppc64le
+rustup target add powerpc64le-unknown-linux-musl
+
+# For linux/s390x
+rustup target add s390x-unknown-linux-musl
+
+# For linux/riscv64
+rustup target add riscv64gc-unknown-linux-musl
+
+# Or install all supported targets at once
+rustup target add \
+    x86_64-unknown-linux-musl \
+    aarch64-unknown-linux-musl \
+    armv7-unknown-linux-musleabihf \
+    arm-unknown-linux-musleabihf \
+    i686-unknown-linux-musl \
+    powerpc64le-unknown-linux-musl \
+    s390x-unknown-linux-musl \
+    riscv64gc-unknown-linux-musl
 ```
 
 #### macOS Cross-compilation Setup
@@ -52,6 +78,9 @@ On macOS, you'll need a cross-compilation toolchain:
 # Install musl cross-compilation tools
 brew install filosottile/musl-cross/musl-cross
 
+# Note: The musl-cross formula typically only includes x86_64 and aarch64 toolchains.
+# For other architectures, you may need additional toolchains or use Docker/remote builders.
+
 # Create a .cargo/config.toml in your project with:
 cat > .cargo/config.toml << 'EOF'
 [target.x86_64-unknown-linux-musl]
@@ -59,6 +88,11 @@ linker = "x86_64-linux-musl-gcc"
 
 [target.aarch64-unknown-linux-musl]
 linker = "aarch64-linux-musl-gcc"
+
+# For other architectures, you'll need to install the appropriate cross-compiler
+# or use cargo-zigbuild which can target all platforms:
+# cargo install cargo-zigbuild
+# Then build with: cargo zigbuild --target <target>
 EOF
 ```
 
@@ -125,6 +159,11 @@ krust build -- --features=prod
 - `linux/amd64` (x86_64-unknown-linux-musl)
 - `linux/arm64` (aarch64-unknown-linux-musl)
 - `linux/arm/v7` (armv7-unknown-linux-musleabihf)
+- `linux/arm/v6` (arm-unknown-linux-musleabihf)
+- `linux/386` (i686-unknown-linux-musl)
+- `linux/ppc64le` (powerpc64le-unknown-linux-musl)
+- `linux/s390x` (s390x-unknown-linux-musl)
+- `linux/riscv64` (riscv64gc-unknown-linux-musl)
 
 ### Multi-Architecture Images
 
@@ -320,9 +359,21 @@ cd krust
 brew install messense/macos-cross-toolchains/x86_64-unknown-linux-musl
 brew install messense/macos-cross-toolchains/aarch64-unknown-linux-musl
 
-# Install Rust targets
+# For full platform support, consider using cargo-zigbuild:
+cargo install cargo-zigbuild
+
+# Install Rust targets (at minimum for tests)
 rustup target add x86_64-unknown-linux-musl
-rustup target add aarch64-unknown-linux-musl  # Optional, for ARM64 support
+rustup target add aarch64-unknown-linux-musl
+
+# For full platform support, add all targets:
+rustup target add \
+    armv7-unknown-linux-musleabihf \
+    arm-unknown-linux-musleabihf \
+    i686-unknown-linux-musl \
+    powerpc64le-unknown-linux-musl \
+    s390x-unknown-linux-musl \
+    riscv64gc-unknown-linux-musl
 
 # Install pre-commit hooks
 pip install pre-commit