forbid chained onbuild, from & maintainer triggers

This changes the way onbuild works:
- forbids the chaining of onbuild instructions
- forbids the use of `onbuild from`
- forbids the use of `onbuild maintainer`

It also makes docker throw errors when encountering such triggers when
executing the triggers during `FROM`.

Three tests have been added:
- ensure that chained onbuild (`onbuild onbuild`) is forbidden
- ensure that `onbuild from` is forbidden
- ensure that `onbuild maintainer` is forbidden

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)