Docker - Building with volume mapping to the workspace fails on errors

[rdar-lab]

When using a local volume to map to the workdir, the bake command fails on errors on creating hard-links:

Exception: subprocess.CalledProcessError: Command 'cp -afl --preserve=xattr ./* /home/build/workspace/build/tmp/sysroots/qemux86' returned non-zero exit status 1

Subprocess output:
cp: cannot create hard link '/home/build/workspace/build/tmp/sysroots/qemux86/lib/libudev.so.1' to './lib/libudev.so.1': Input/output error
cp: cannot create hard link '/home/build/workspace/build/tmp/sysroots/qemux86/usr/lib/libudev.so' to './usr/lib/libudev.so': Input/output error

[rdar-lab]

As a workaround:

1. Do not map the volume with the source code to the container, instead:
   a. Create a persistent docker container with a specific name
   b. Copy the source files to the /home/build/workspace directory (using the docker cp command for example)
   c. Start the build process
   d. If needed copy back the artifacts of the build (Again using docker cp command)
   e. Remove the container

[rdar-lab]

This does not happen when using local volume mapping to the 'tmp' directory. The README.md file was updated with the necessary steps.