registry: make certain headers optional

For a pull-only, static registry, there only a couple of headers that
need to be optional (that are presently required.
* X-Docker-Registry-Version
* X-Docker-Size
* X-Docker-Endpoints

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
registry: Info collection

roll version and standalone information into the _ping. And to support
Headers they are checked after the JSON is loaded (if there is anything
to load). To stay backwards compatible, if the _ping contents are not
able to unmarshal to RegistryInfo, do not stop, but continue with the
same behavior.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
typo fix: 'methid' -> 'method'
Fix container.json sample to be loadable by nsinit.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Fix typos in nsinit logs.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Add enabled option to namespaces and capabilities spec in
container.json. Although we don't yet check for enabled everywhere.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Improved README formatting.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Add memory usage and max usage stats.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Updated sample config and README to match the default template for
native execdriver.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Updated sample config to be usable. We should change the namespace
config to not need "value" later.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
static_registry: update the test for the new struct

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Adding a test for blkio stats.

Also adds a test utility we can use for other cgroup tests.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Fix bug in engine.Sender

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine/spawn: run an engine in a subprocess, remote-controlled by Beam

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine.Len returns the number of keys in an env

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Docs/CSS fix: Aallow viewport expand to user's preference

This commit removes the "max-width" property of DOM "containers";
Thus letting the viewport to expand to fill the available space.

This commit aims to bring pleasure to Docker docs' readers' eyes,
And to make them happy by letting them profit more from their large
monitors. (or use more efficiently their smaller ones).

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Docs/theme/MAINTAINERS: Modify/Update list of MAINTAINERS

So far it has been mostly my duty to create and maintain docs' design/theme.

This commit adds myself to the list of maintainers by modifying
the MAINTAINERS file under the docs/theme directory.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Docs/CSS: Amend code block rendering.

This commit aims to improve the rendering of code blocks by
reducing the padding, matching the font-size with the rest of the
documentation text and finally, by changing the background colour
back to white from its current gray-ish state which matches the
background colour and making it really hard to spot the code.

Note: The BG colouring issue is due to converting the main BG
to gray whilst missing to change parts that where gray to white, i.e.,
not making the complete switch.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Docs/CSS: Fix Notes/Warnings blocks' BG colour to improve its state.

Docs' BG was original "#fff" and blocks such as code blocks or
warning-notes blocks were coloured in #F2F2F2.

In order to make it easier to read everything, the BG colour was
changed to #fff. However, the switch missed to convert other blocks'
BG colour.

This commit aims to re-introduce the correct contrast by changing the
BG colour of warning/notes block to #fff.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5431 from shykes/engine-len

engine.Len returns the number of keys in an env