Got it. It's basically just the default SPM project structure with the dependencies that my project is using (the main.swift file is completely empty).
Running this Docker command results in the error:
docker run \
--rm \
--volume "$(pwd)/:/src" \
--workdir "/src/" \
swift:5.5.3-amazonlinux2 \
/bin/bash -c "swift build --product LinkerErrorExample -c release --static-swift-stdlib"