Chaining FROM ./path in Dockerfile

[lk4d4math@gmail.com: add support for `FROM ./path`, `FROM git://url` and `FROM http://url/to/Dockerfile`]
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

[teabee89@gmail.com: remove git:// and dockerfile-url support for Dockerfile FROM keyword]
[teabee89@gmail.com: rebase + gofmt]
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
The \`<path>\` must be a directory within current build's context that contains
a \`Dockerfile\`. Note that the leading slash, \`./\`, is necessary if you want to specify the \`<path>\` option. This will build the image specified in the `\./<path>/Dockerfile\` and when that build is complete use the resulting image as the source of the `\FROM\` instruction.