This commit introduces the Docker-based local WordPress development environment to the 4.3 branch.
Merges [44176,45445,45745,45762,45783-45784,45800,45819,45885,46320,46999,47225,47912,48121,49267,49335,49358,49360,49362] to the 4.3 branch.
See #48301, #47767.
git-svn-id: https://develop.svn.wordpress.org/branches/4.3@50245 602fd350-edb4-49c9-b593-d223f7449a82