docker: add patch coverage utilities to Quibble
The mediawiki/phpunit-patch-coverage package is an utility to generate
coverage reports for a git patch. It does not belong to MediaWiki and
we thus ship it in the container for Quibble.
We only generate report with php 7.0, so it is only needed in
Use a temporary directory as composer home for cache etc.
Install phpunit-patch-coverage in its own directory under /opt and link
the binary to /usr/local/bin so it can be find via PATH.
Provide shell wrappers to be used instead of Quibbleto be used instead
of Quibble stages (via --commands).