Deprecate 'docker insert'

'docker insert' is an old command which predates 'docker build'. We no
longer recommend using it, it is not actively maintained, and can be
replaced with the combination of 'docker build' and 'ADD'.

This removes the command from usage and documentation, and prints a
warning when it is called.

The command still works but it will be removed in a future version.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #5005 from shykes/deprecate-insert

Deprecate 'docker insert'