If you look carefully on the output of your building you can see this message:
Sending build context to Docker daemon 9.728kB
This is where ALL your files from the build context (in your case /data/docker/test with everything inside) is transferred to the docker daemon.
Now, don’t change your docker file, but add into anywhere inside /data/docker/test 10 another zip files with size 2Gb each, and re-run the build, and see what will happen to “Sending build context to Docker daemon”.