The file Dockerfile.crossbuild is used to generate the Docker image quay.io/geotrellis/gdal-warp-bindings-crossbuild which contains the cross-compilers needed to generate Macintosh and Windows shared libraries.
This image is based heavily on Manfred Touron's multiarch/crossbuild image (in fact, it is just that image with non-x86 compilers removed, some unneeded packages removed, and based on Ubuntu 20.04 instead of Debian Jessie).
An important legal note from that project's README.md concerning Apple's OSX SDK is reproduced below:
OSX/Darwin/Apple builds: Please ensure you have read and understood the Xcode license terms before continuing.
The file Dockerfile.environment-amd64 is used to generate the Docker images found under quay.io/geotrellis/gdal-warp-bindings-environment that contain amd64 compilers. The Docker images with amd64 images are derived from quay.io/geotrellis/gdal-warp-bindings-crossbuild. The Dockerfile mentioned above adds Macintosh, Windows, and amd64 Linux versions of OpenJDK and GDAL.
The file Dockerfile.environment-arm64 is used to generate the Docker images found under quay.io/geotrellis/gdal-warp-bindings-environment that contain arm64 compilers. The arm64 image is not derived directly from a Debian Buster base image and contains a aarch64-gnu-linux toolchain as well as OpenJDK and GDAL.
The Docker files in this directory are MIT Licensed.