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)
Adding first draft of Official Repo Guidelines

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Fix gh#7747 - filter bare newline output and log client pull to stdout

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
Revisions based on feedback.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Format and typo fixes

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
More format fixes.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
More format wrestling.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Fix formatting, change to markdown block for long

Signed-off-by: Joe Ferguson <joe@infosiftr.com>
Merge pull request #1 from yosifkit/adding_official-repo-guidelines

Fix formatting, change to markdown block for long
Fixes a couple small typos

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge branch 'adding_official-repo-guidelines' of https://github.com/fredlf/docker into adding_official-repo-guidelines

Conflicts:
	docs/sources/docker-hub/official_repos.md

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Add docker-exec to main manpage

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Try to avoid issues when the Docker daemon restarts.

This change will allow the Docker daemon's init script to wait up to 5
minutes before being forcibly terminated by the initscript. Many
non-trivial containers will take more than the default 3 seconds to
stop, which can result in containers whose rootfs is still mounted and
will not restart when the daemon starts up again, or worse, orphan
processes that are still running.

Signed-off-by: Steven Merrill <steven.merrill@gmail.com>
tarsum: allow for alternate block ciphers

Not breaking the default cipher and algorithm for calculating checksums,
but allow for using alternate block ciphers during the checksum
calculation.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Add support for copy/add with multiple src files

Part one of solution for issue #6820

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8091 from jfrazelle/exec-main-manpage

Add docker-exec to main manpage
Automatically clean up hello-world container with --rm

Signed-off-by: Bruno Binet <bruno.binet@gmail.com>
Merge pull request #8076 from duglin/MultiCopyAdd

Add support for copy/add with multiple src files
FIX 6613:launch docker fail with space named drive

Signed-off-by: Guillaume Dufour <guillaume.duff@gmail.com>
Signed-off-by: Tibor Vass <teabee89@gmail.com>
Fix detaching from attached container

Text was copied from man-pages https://raw.githubusercontent.com/docker/docker/master/docs/man/docker-attach.1.md

Signed-off-by: Jyrki Puttonen <jyrkiput@gmail.com>