Fixes CVE-2019-13139 git ref cmd injection

[TomSweeneyRedHat]

Signed-off-by: TomSweeneyRedHat tsweeney@redhat.com

- What I did
Add a check to the git reference to validate that it did not have a leading dash '-' in front of it, allowing for a security injection. This is per CVE-2019-13139 and BZ https://bugzilla.redhat.com/show_bug.cgi?id=1733941

Corresponding change from upstream Docker: moby#38944

Fixed in Docker 1.13.1-rhel here: #359

- How I did it
Lots of vi and testing.

- How to verify it
Without the fix this is the way to reproduce the issue:

# rm -rf /tmp/docker-build-git* 
# docker build 'git://github.com/RedHatOfficial/openhardware#-btest-branch:test'  # ignore any errors
# cd /tmp/docker-build-git* 
# git branch 

With the error in play, there will be a master and a rogue test branch created, the test branch should not be there.

With the fix:

# rm -rf /tmp/docker-build-git* 
# docker build 'git://github.com/RedHatOfficial/openhardware#-btest-branch:test'  # ignore any errors
unable to prepare context: unable to 'git clone' to temporary context directory: invalid refspec: -btest-branch
# cd /tmp/docker-build-git* 
# git branch 

and only the master branch should be there.

- Description for the changelog

Addresses CVE-2019-13139 - docker: command injection due to a missing validation of the git ref command

- A picture of a cute animal (not mandatory but encouraged)

[rhatdan]

LGTM