Updated the PHP documentation build environment to use a docker

container, holding the specific version of ApiGen known to produce
decent results.

This is necessary because that version of ApiGen requires PHP 5 (it
doesn't work with PHP 7), and newer versions of ApiGen are currently
completely broken, and don't support half the options necessary to
produce decent output.

The docker container is based on alpine (just about the smallest
linux distro available), with the required PHP 5 packages, and ApiGen
copied in.

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Imported using git-svn from
https://dev.csi.cam.ac.uk/svn/lookup/trunk@101