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)
Add "validate" Makefile target

This was supposed to be part of my previous PR, but somehow got missed.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
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.
Merge pull request #5249 from tianon/make-validate

Add "validate" Makefile target