DCT-JS should support ONBUILD instruction

[TyIsI]

Not sure how I missed that one.

Limitations:

• Chaining ONBUILD instructions using ONBUILD ONBUILD isn't allowed.
• The ONBUILD instruction may not trigger FROM or MAINTAINER instructions.
• ONBUILD COPY --from is not supported.

Notes:

• Restrict commands passed (exclude: ONBUILD, FROM, MAINTAINER)
• Guard from flag to non-stage parameters (do this blank for every from; it's a fair assumption that if it breaks COPY, that it will break RUN mount from if it refers to a stage)