841 - docs: warn about the transmission of data to the docker daemon & ADD without context

This updates the documentation to mention that:
1. a lot of data may get sent to the docker daemon if there is a lot of
data in the directory passed to docker build
2. ADD doesn't work in the absence of the context
3. running without a context doesn't send file data to the docker
daemon
4. explain that the data sent to the docker daemon will be used by ADD
commands

updated:
5. explain that using an absolute path with docker build will only transfer the data needed by the ADD commands found in the Dockerfile to be found in that directory

This PR fixes #841.