feat: Add multi-arch support

- Support building for multiple platforms with --platform flag
- Accept comma-separated platforms or multiple --platform flags
- Default to building for linux/amd64 and linux/arm64
- Add ARM runner (ubuntu-24.04-arm) to CI matrix
- Create manifest list for multi-arch images (TODO: push to registry)
- Update integration tests to handle multi-platform builds
- Add cross-compilation setup instructions to README
- Add .cargo/config.toml for local development

Breaking changes:
- Default behavior now builds for multiple platforms (amd64+arm64)
- Use --platform linux/amd64 to build for single platform

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-latest, ubuntu-24.04-arm]
         rust: [stable, beta]
     steps:
     - uses: actions/checkout@v4
@@ -24,12 +24,17 @@ jobs:
       uses: dtolnay/rust-toolchain@master
       with:
         toolchain: ${{ matrix.rust }}
-        targets: x86_64-unknown-linux-musl
-    - name: Install cross-compilation tools (Ubuntu)
+        targets: x86_64-unknown-linux-musl,aarch64-unknown-linux-musl
+    - name: Install cross-compilation tools (Ubuntu x64)
       if: matrix.os == 'ubuntu-latest'
       run: |
         sudo apt-get update
-        sudo apt-get install -y musl-tools
+        sudo apt-get install -y musl-tools gcc-aarch64-linux-gnu
+    - name: Install cross-compilation tools (Ubuntu ARM)
+      if: matrix.os == 'ubuntu-24.04-arm'
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y musl-tools gcc-x86-64-linux-gnu
     - name: Cache cargo registry
       uses: actions/cache@v4
       with:
@@ -105,11 +110,11 @@ jobs:
     - name: Install Rust
       uses: dtolnay/rust-toolchain@stable
       with:
-        targets: x86_64-unknown-linux-musl
+        targets: x86_64-unknown-linux-musl,aarch64-unknown-linux-musl
     - name: Install musl tools
       run: |
         sudo apt-get update
-        sudo apt-get install -y musl-tools
+        sudo apt-get install -y musl-tools gcc-aarch64-linux-gnu
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v3
     - name: Build krust
@@ -136,6 +141,10 @@ jobs:
       run: |
         cd example/hello-krust
         krust build --no-push --image local.test/hello:latest ./
+    - name: Test multi-arch build
+      run: |
+        cd example/hello-krust
+        krust build --no-push --platform linux/amd64,linux/arm64 --image local.test/multiarch:latest ./
 
   # Takes too long to run on CI, so it's commented out for now.
   # coverage: