This commit merges the workflow files required to run automated testing on GitHub Actions.
In addition, [49836] and [50285] have been included in order to keep the local Docker environment consistent across all branches.
Merges [49162,49168-49169,49175,49204,49227-49228,49244,49369,49371,49548,49781-49784,49786,49836,49938,50268,50285,50298] to the 4.3 branch.
See #50401.
git-svn-id: https://develop.svn.wordpress.org/branches/4.3@50317 602fd350-edb4-49c9-b593-d223f7449a82
