remove symlinks before recreating `/run` directory.

busybox symlinks /run to /tmp. Delete symlink and give /run it's own
real directory before recreating to Ubuntu specs.
Use "IDLE" program for keeping volume containers alive.

IDLE is `tail -f /dev/null`. From what I can gather, it uses no CPU, has
no output, and seems to be safe.

fixes #1
Refactor to use entrypoint script.

The new script takes over the following tasks from elsewhere:
- ENV checking
- wheel repository pulling via git
- file permission setting

It adds the following features:
- "IDLE" final program for keeping containers running with no overhead
- graceful container restarts
- an UPDATES env var option that will:
    - update the existing Wheel repositories when the container is
      restarted and
    - enable the addition of others at runtime via `docker run -e`
- support for multiple WHEEL_REPO env vars delivered through the
  /build-env file at build time, through `docker run -e` at run time (in
  both dynamic and static modes), or any combination of the two.

fixes #3
Update comments and instructions. Mention .dockerignore.

Update lots of comments everywhere.

The actual .dockerignore changes got eaten up by another commit, but the
instructions were updated here. The big change is that now the Hub will
upload the entire context, catching any combination of /config, /data,
folders and other files (build-env) that we put there. This was more
flexible and easier to design for now that we can blacklist items from
the context.

The original reason for not doing this was phantom image size
creep when uploading the entire context. The image seemed to grow by
more then what was in the context. That seems to have disapeared now.