add --releasever=/ to mkimage-yum.sh

I didn't realize the commit required a Docker-DCO so it failed the travis-ci build. So I removed the commit from my forked repo. Now it looks like there is a pull request with no commit. So here it is again:

Needed to add '--releasever=/' flag to run yum groupinstall on Centos7 (didn't try on anything else). This snippet from yum man page explains why:

```
Note: You may also want to use the option --releasever=/ when creating the installroot as otherwise the $releasever value is taken from the rpmdb within the installroot (and thus.  will  be  empty,  before  creation).
```

Docker-DCO-1.1-Signed-off-by: Matt Schurenko <matt.schurenko@gmail.com> (github: mschurenko)
devmapper: include dm_deps information in debug

Signed-off-by: Vincent Batts <vbatts@redhat.com>
daemon logging: unifying output and timestamps

A little refactor of the ./pkg/log so engine can have a logger instance

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Adding self to various maintainers files.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Implementing IPv6 functionality for ipallocator

Closes #6975

Signed-off-by: Malte Janduda <mail@janduda.net>
Add more names

Docker-DCO-1.1-Signed-off-by: Brian Goff <bgoff@cpuguy83-mbp.home> (github: cpuguy83)
pkg/version: lint and add comments
docker save: Do not save to a terminal.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
pkg/truncindex: lint and add comments

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
pkg/units: lint

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
pkg/timeutils: lint and add comments

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
registry: lint

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
pkg/graphdb: some linting

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Add a best practice to reduce cache invalidations

inspired by https://github.com/docker-training/docker-fundamentals/pull/206

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>
docs: Remove `Detach` parameter from Exec Create example

Signed-off-by: Fabio Rehm <fgrehm@gmail.com>
merged conflicting commits

Signed-off-by: Nathan Hsieh <hsieh.nathan@gmail.com>
Squashed commit of the following:

commit 3f9e9185fe7ee51f2fe55f0fc0d4fffdc4f289dc
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Mon Oct 6 10:57:35 2014 -0700

    changed the title of back buttons

commit f86934424e85931ec293e711ceaa93ee920828fb
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Mon Oct 6 10:53:15 2014 -0700

    Finished level 2, refactored tests

commit 8f502bce05293cccaf200b69ce5f5826eee72484
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Mon Oct 6 08:52:37 2014 -0700

    stylized better and added level 2

commit 07b2276b346c34c0cc0faa57500c40e120e77888
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Fri Oct 3 18:17:29 2014 -0700

    broke tutorial tests styles

commit 35d84147dc2f65b0ffeea5faf304add903219b1e
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Fri Oct 3 17:23:48 2014 -0700

    structured test level1 md file

commit 808d01b0d55d67eb1017f290a29da6c7d38565f2
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Fri Oct 3 17:15:46 2014 -0700

    Modified files and integrated tutorial through new page

commit 0f0093f2882489c3eeb6f8870f2b8aa64dc939a3
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Thu Oct 2 14:34:44 2014 -0700

    more refactoring

commit 5a9b98e55ebd455ccf2c0ced20f984545a0b6d71
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Thu Oct 2 11:46:58 2014 -0700

    clean js code

commit af3bbd8d5e1dffdaa1780f83b909ff566906e513
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Thu Oct 2 11:26:39 2014 -0700

    level 1 cleanup

commit c4852a7766ab4fbd978d65c8352ace05eb427ef5
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Thu Oct 2 11:20:17 2014 -0700

    fixed up fill-in level1

commit 7f02d80942549dec9c05f784b777fcb32d5bb81b
Author: Nathan Hsieh <hsieh.nathan@gmail.com>
Date:   Wed Oct 1 17:45:42 2014 -0700

    added dockerfile tut, stylized lesson1 Questions

Signed-off-by: Nathan Hsieh <hsieh.nathan@gmail.com>
added fixes to address comments

Signed-off-by: Nathan Hsieh <hsieh.nathan@gmail.com>
Merge pull request #8461 from SvenDowideit/copy-specific-files-from-context

Add a best practice to reduce cache invalidations
Merge pull request #8419 from nhsiehgit/dockerfiletut

Dockerfiletut