Add TF_PACKAGE_VERSION to Dockerfiles that install via pip

This change adds `--build-arg TF_PACKAGE_VERSION=...` to the standard
Dockerfiles. It also picks up a tiny --build-arg documentation typo.

I also merged the dockerfile specs into one spec. Since the Dockerfiles
are usually going to be built all at once (and the default behavior of
the assembler is to wipe the build directory), it seems more convenient
to put them all in one place.