Engine: optional Logging field to disable custom logging in engine.

* The default behavior is preserved
* This disables the use of the `TEST` environment variable in engine.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Mark notes and warnings, fix broken link renderings within.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5307 from ostezer/docs-amend-notes-warnings

Mark notes and warnings, fix broken link renderings within.
Introducing spīritus lēnis to fix code highlighting issues due to apostrophe.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #4166 from shykes/engine-logging

Engine: optional Logging field to disable custom logging in engine.
Fix footer links. They are pointing to the wrong direzione.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5311 from ostezer/docs-fix-footer-links

Fix footer links. They are pointing to the wrong direzione.
Merge pull request #5309 from ostezer/docs-fix-apostrophe

Introducing spīritus lēnis to fix code highlighting issues due to apostrophe.
make scratch a special case for build and pull

This commit makes `from scratch` a special case for `docker build` and
it makes Docker not pull the image with the scratch ID.

In both cases, it will just create an empty image with the same ID if
it doesn't exist already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
docs: explain scratch & where it's blocked

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)