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)
static_registry: update the test for the new struct

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
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)
Update some whitespace in hack/make/test-integration-cli for consistency

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Use "docker load" to create "scratch" in hack/make/test-integration-cli (instead of implicitly pulling it from the index)

Creating the "docker save" tarball for "scratch" is pretty simple.  I've also extrapolated the "docker build -t busybox ." logic into a separate "hack/make/.ensure-busybox" file so that it can eventually be reused easier.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Use proper scheme with static registry
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4607 from vbatts/vbatts-static_registry

static registry support
Cleanup existing controllers when cleanup fails mid-way.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #5474 from crosbymichael/use-proper-scheme

Use proper scheme with static registry
docs: DisableNetwork -> NetworkDisabled

Docker-DCO-1.1-Signed-off-by: Samuel Reis <srijs@airpost.net> (github: srijs)
Merge pull request #5476 from rjnagal/libcontainer-fixes

Cleanup cgroups on Set failures
Merge pull request #5475 from srijs/patch-1

docs: DisableNetwork -> NetworkDisabled
Update working-with-docker.md
Merge pull request #5478 from amangoel/patch-1

Update working-with-docker.md
Merge pull request #5460 from tianon/no-pull-scratch

Use "docker load" to create "scratch" in hack/make/test-integration-cli (instead of implicitly pulling it from the index)
Merge pull request #5440 from ostezer/docs-improve-code-block-rendering

Docs/CSS: Amend code block rendering.