Fix bug where lowercase FROM instruction was ignored

I forgot that Dockerfile instructions are case-insensitive, especially because lowercase looks so silly, but this makes "hipache" build when it should be deferred to only build after "ubuntu".

Also, awk's default is to split on any number of whitespace-chars, so no need for the explicit "-F" there.
Merge pull request #231 from infosiftr/lowercase-from

Fix bug where lowercase FROM instruction was ignored