Clarify USER instruction documentation

Reuse WORKDIR wording to specify that the USER instructions affect the
following RUN, CMD, and ENTRYPOINT instructions.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
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>
Update logrus to v0.6.0 so we can use ParseLevel() for PR #8335

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8907 from icecrime/clarify_user_instruction_doc

Clarify USER instruction documentation
Merge pull request #8932 from duglin/UpdateLogrus

Update logrus to the latest so we can use ParseLevel() for PR #8335
Merge pull request #8739 from duglin/CleanupDockerIgnores

Do some cleanup on .dockerignore paths