This is a reproducible Dockerfile for cross compiling for Apple platforms from Linux, usable as base image for CI style setups.
This builds on cctools-port for providing the basic tools for cross compilation, wrapping it up into a easy to use and reproducible Dockerfile.
This setup currently uses a vanilla LLVM/Clang release instead of the Apple provided clang sources.
This requires an original Xcode.app bundle (which can't be redistributed freely). It has been tested with and should work with Xcode versions 11 and newer. Older versions of Xcode can be downloaded from Apple.
The Xcode bundle can be stripped down to more manageable sizes for use with this cross compilation setup, since very little of the bundle actually is used:
cp -a /Applications/Xcode.app .
./strip-xcode.sh Xcode.app
tar -Jcvf xcode.tar.xz Xcode.app
rm -rf Xcode.app
Host the resulting xcode.tar.xz file somewhere, then build the docker image like this:
docker build --build-arg XCODE_URL=http://path/to/your/xcode.tar.xz .
The docker image has got all the necessary tools added to the path, but
at the end of the path, so that e.g. gcc still resolves to the normal
host compiler (for cases when projects need to compile things for both
the host and the target). The cross tools can be used with normal cross
compilation prefixes like x86_64-apple-darwin-gcc (which invokes clang,
not gcc), or less architecturally biased like apple-darwin-gcc (for cases
when the architecture will be specified with an -arch parameter.