Pin image to specific version so that we don't get surprise failures
down the line.
Use common action to configure the default PHP version (the matrix
versions only affect test running).
Update PHP versions to supported versions in this image.
Two workflows have been added,
* PHP build - Runs tests on version 7.2, 7.3, also runs phpcs
* Node.js build - Runs tests on version 8.x, 10.x, 12.x, also
runs ESLint
Bug: T218639