Do some cleanup on .dockerignore paths

While working on the fix for #8330 I noticed a few things:
1 - the split() call for the .dockerignore process will generate a blank
    "exclude".  While this isn't causing an issue right now, I got worried
	that in the future some code later on might interpret "" as something bad,
	like "everything" or ".".  So I added a check for an empty "exclude"
	and skipped it
2 - if someone puts "foo" in their .dockerignore then we'll skip "foo".
    However, if they put "./foo" then we won't due to the painfully
	simplistic logic of go's filepath.Match algorithm.  To help things
	a little (and to treat ./Dockerfile just like Dockerfile) I added
	code to filepath.Clean() each entry in .dockerignore.  It should
	result in the same semantic path but ensure that no matter how the
	user expresses the path, we'll match it.

Signed-off-by: Doug Davis <dug@us.ibm.com>
You can replace file in current `fakeContext` with `Add` method.