Create diagnostics for invalid ONBUILD triggers

[rcjsuen]

FROM, MAINTAINER, and ONBUILD instructions are not allowed to follow an ONBUILD. We should create an error diagnostic if this case is detected.

FROM scratch
ONBUILD onbuild

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ONBUILD onbuild
Chaining ONBUILD via `ONBUILD ONBUILD` isn't allowed

FROM scratch
ONBUILD maintainer

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ONBUILD maintainer
MAINTAINER isn't allowed as an ONBUILD trigger

FROM scratch
ONBUILD from

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ONBUILD from
FROM isn't allowed as an ONBUILD trigger