Add more info about when build cache is invalidated/used - Issue #3636

Closes #3636

Signed-off-by: Doug Davis <dug@us.ibm.com>
Maybe simpler wording for "during the process" and a short introduction *that* Docker has a cache (although I haven't checked the style of the rest of this document), something like;

"Docker features a build cache to speed-up building images. When building an image from a Dockerfile, Docker will ..."

But I'll leave the decision on "style" to James and Fred.