caused by https://github.com/wikimedia/mediawiki-vendor/commit/58b70226d77dcd20e85359f198269aa720c14530 merged by @cscott 8 hours ago. Started failing at 23:30 UK time.
06:20:01 wikimedia/parsoid: 0.19.0-a16 installed, 0.19.0-a15 required.
06:20:01
06:20:01 Error: your composer.lock file is not up to date. Run "composer update --no-dev" to install newer dependencies