Add note about paths to docker section of README

Right now, the docs make no mention of needing to pay attention to where the user is running the supplied docker create command. This can confuse users if they are not familiar with how docker build works or what its arguments mean.

This will correct that lack of direction while preserving the current workflow based on relative paths. I felt it was too disruptive and close-minded to direct users to clone to a specific path, as everyone has different prefs for their local machine.

Signed-off-by: Cullen Taylor ctaylor@gitlab.com