Make `FROM` optional for base image Dockerfiles

There has been a lot of discussion (issues 4242 and 5262) about making
`FROM scratch` either a special case or making `FROM` optional, implying
starting from an empty file system.

This patch makes the build command `FROM` completely optional from now on
and if used requires it to be the first command in a Dockerfile. It DOES
NOT implicitly pull or install the image with ID '511136ea3c5a64f264b7...'
but instead makes it possible for a container to exist with a blank image
ID. This means every image ever can now use one fewer layer!

Goodbye '511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158',
it was nice knowing you.

Fixes #4242

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)