runconfig.Config.AsScript returns a text representation of the Config in the Dockerfile syntax

This allows easy conversion of a Config object to and from the Dockerfile syntax.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
pkg/dockerfile: a simple parser for the original Dockerfile format

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
pkg/dockerfile.ReflectorHandler: gracefully ignore unknown instructions (and print a warning)

This makes pkg/dockerfile usable as the reference parser for 'docker build'.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

Conflicts:
	runconfig/script.go
runconfig.Config.AsScript returns a text representation of the Config in the Dockerfile syntax

This allows easy conversion of a Config object to and from the Dockerfile syntax.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)