Updated Helium Dockerfile to reduce size of image.

- Squash a few steps to reduce image size by 229MB.
- Functionality of image is unchanged.
- Slightly better docs in Dockerfile.

Change-Id: I22706b2699d5610a56c091a0f9bf2b88c27a7316
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>