builder: Update documentation WRT environment replacement

@fredlf @SvenDowideit @jamtur01 @tiborvass 
There are too many sandboxing issues with environment replacement — I made a semi-executive decision to exclude them for now.

> On Oct 27, 2014, at 4:14 PM, Sebastiaan van Stijn <notifications@github.com> wrote:
> 
> In docs/sources/reference/builder.md:
> 
> > +    WORKDIR ${foo}   # WORKDIR /bar
> > +    ADD . $foo       # ADD . /bar
> > +    COPY \$foo /quux # COPY $foo /quux
> > +
> > +The instructions that handle environment variables in the `Dockerfile` are:
> > +
> > +* `ENV`
> > +* `ADD`
> > +* `COPY`
> > +* `WORKDIR`
> > +* `EXPOSE`
> > +* `VOLUME`
> > +* `USER`
> > +
> > +`ONBUILD` instructions are **NOT** supported for environment replacement, even
> > +the instructions above.
> Nothing to stop this from being merged, but reading this, I wonder why and perhaps this needs some extra clarification; is this by design? Not implemented (yet)? Not supported because of possible security implications?
> 
> A bit of explanation may prevent needless support tickets and/or discussions.
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/8807/files#r19444373>.
>