Linking containers added to API example

Linking containers was not covered in the API documentation. I looked it up in the client source code and testing it manually.
Remove "production ready" from the roadmap

It's done as of v1.0.0.

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)
Adds info on how to get env vars from container

Currently the docs just say "let's go into the container and look at the
env vars", but doesn't show how to do it.  It is also not currently
possible to enter the container as it is written in the doc
( container started with app.py and then somehow later on we have a
shell in the same container )

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
Merge pull request #6307 from W4RH4WK/master

restore logo for README.md
add test

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
add hostname generation with --net=host

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6365 from vieux/allow_net_none_h

Allow --net=none and -h
Merge pull request #6360 from vieux/remove_beam

remove unused beam, will be back later as libchan
Merge pull request #6381 from glyn/prod-ready

Remove "production ready" from the roadmap
Merge pull request #6391 from vieux/fix_hostname_file_net_host

fix hostname generation with --net=host
Merge pull request #6214 from LK4D4/fix_some_more_race_conditions

Fix some more race conditions
Add support for client certificates for registries

This lets you specify custom client TLS certificates and CA root for a
specific registry hostname. Docker will then verify the registry
against the CA and present the client cert when talking to that
registry.  This allows the registry to verify that the client has a
proper key, indicating that the client is allowed to access the
images.

A custom cert is configured by creating a directory in
/etc/docker/certs.d with the same name as the registry hostname. Inside
this directory all *.crt files are added as CA Roots (if none exists,
the system default is used) and pair of files <filename>.key and
<filename>.cert indicate a custom certificate to present to the registry.

If there are multiple certificates each one will be tried in
alphabetical order, proceeding to the next if we get a 403 of 5xx
response.

So, an example setup would be:
/etc/docker/certs.d/
└── localhost
    ├── client.cert
    ├── client.key
    └── localhost.crt

A simple way to test this setup is to use an apache server to host a
registry. Just copy a registry tree into the apache root, here is an
example one containing the busybox image:
  http://people.gnome.org/~alexl/v1.tar.gz

Then add this conf file as /etc/httpd/conf.d/registry.conf:

 # This must be in the root context, otherwise it causes a re-negotiation
 # which is not supported by the tls implementation in go
 SSLVerifyClient optional_no_ca

 <Location /v1>
 Action cert-protected /cgi-bin/cert.cgi
 SetHandler cert-protected

 Header set x-docker-registry-version "0.6.2"
 SetEnvIf Host (.*) custom_host=$1
 Header set X-Docker-Endpoints "%{custom_host}e"
 </Location>

And this as /var/www/cgi-bin/cert.cgi

 #!/bin/bash
 if [ "$HTTPS" != "on" ]; then
     echo "Status: 403 Not using SSL"
     echo "x-docker-registry-version: 0.6.2"
     echo
     exit 0
 fi
 if [ "$SSL_CLIENT_VERIFY" == "NONE" ]; then
     echo "Status: 403 Client certificate invalid"
     echo "x-docker-registry-version: 0.6.2"
     echo
     exit 0
 fi
 echo "Content-length: $(stat --printf='%s' $PATH_TRANSLATED)"
 echo "x-docker-registry-version: 0.6.2"
 echo "X-Docker-Endpoints: $SERVER_NAME"
 echo "X-Docker-Size: 0"
 echo

 cat $PATH_TRANSLATED

This will return 403 for all accessed to /v1 unless *any* client cert
is presented. Obviously a real implementation would verify more details
about the certificate.

Example client certs can be generated with:

openssl genrsa -out client.key 1024
openssl req -new -x509 -text -key client.key -out client.cert

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Fix typo
Merge pull request #6411 from alazaro/patch-1

Fix typo
Increase time before exit in TestMultipleAttachRestart

Sometimes third attacher attaching to already stopped container.
Also I've changed prefix to attach and fixed cleanup on Fatal.
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
docs: add spotify docker-client link

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
Add helper function to make prctl system call.
Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)
Merge pull request #6416 from danielnorberg/dano/docs-spotify-client

docs: add spotify docker-client link
Note user name and password in docs on boot2docker

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathan.leclaire@docker.com> (github: nathanleclaire)
Merge pull request #6421 from nathanleclaire/doc_fix

Note user name and password in docs on boot2docker