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)
graph: add VirtualSize for inspect output

Currently inspect output just shows Size info, and that usally be
very small and even 0 which is confusing.

Fixes #8016

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Improve error for docker exec & LXC

Signed-off-by: Victor Vieux <vieux@docker.com>
Rewrite TestBuildForceRm to not use fixtures

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Rewrite TestBuildHistory to not use fixtures

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Fix code example formatting in best-practices article

Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com>
Rewrite TestBuildRm to not use fixtures

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Rewrite TestBuildWithInaccessibleFilesInContext to not use fixtures

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Rewrite TestContextTar tests to not use fixtures

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Fix quoting of TESTFLAGS in devenvironment.md

Copy and pasting the example should now work, rather than TESTFLAGS being silently ignored.
Merge pull request #8207 from LK4D4/rewrite_more_fixtures_test

Rewrite more fixtures test to not use fixtures.
Replace get.docker.io -> get.docker.com and test.docker.io -> test.docker.com

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Merge pull request #8195 from tiborvass/migrate-get.docker.io

Replace get.docker.io -> get.docker.com and test.docker.io -> test.docker.com
Merge pull request #8212 from hugoduncan/patch-1

Fix quoting of TESTFLAGS in devenvironment.md
Merge pull request #8205 from sequenceiq/master

Fix code example formatting in best-practices article
Updated output from the docker cli help

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Provide full hostname with domainname to underlying container layer
Addresses #7851

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
devmapper: include dm_deps information in debug

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Add checks for app/json - issue #2515

Signed-off-by: Doug Davis <dug@us.ibm.com>
builder: Fix handling of VOLUME command where multiple volumes are
specified in a space delimited list.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)