Bug 1969719: add php-intl extension to docker

Test by standing up via docker and pointing a browser at it. Looking at
ensure_sanity() in lib/mahara.php this should never have been able to work.

Change-Id: Idcb6e50d0172c20a9b7e6af2f5476772fe2cef92
Signed-off-by: Gold <gold@catalyst.net.nz>
(cherry picked from commit e1ec09bd)