Add cross-compilation support for Linux and Windows

- Add Docker-based build system for cross-compiling on Apple Silicon
- Create Dockerfiles for Linux x86_64 and ARM64, Windows x86_64
- Add build-all.sh script for automated multi-platform builds
- Configure cargo-deb and cargo-generate-rpm for Linux packaging
- Update README with cross-compilation instructions

Supported outputs:
- Linux x86_64: .deb, .rpm, .tar.gz
- Windows x86_64: .zip

Note: Linux ARM64 disabled due to upstream zune-jpeg NEON bug

Cargo.toml

@@ -41,3 +41,17 @@ open = "5"
 [profile.release]
 opt-level = 3
 lto = true
+
+[package.metadata.deb]
+maintainer = "cottongin"
+copyright = "2026"
+depends = "ffmpeg"
+section = "graphics"
+# Note: cargo-deb auto-detects binary location when --target is specified
+
+[package.metadata.generate-rpm]
+assets = [
+    { source = "target/x86_64-unknown-linux-gnu/release/avif-maker", dest = "/usr/bin/avif-maker", mode = "755" }
+]
+[package.metadata.generate-rpm.requires]
+ffmpeg = "*"