Use Docker image for acceptance tests

The Docker image for acceptance tests provides support for both the PHP
built-in web server and the Apache web server; the acceptance tests for
the server are run on the PHP built-in web server, but the acceptance
tests for some apps will have to be run on the Apache web server (for
example, Talk, as it uses long polling), so a Docker image to support
both cases has to be used in "run.sh". ".drone.yml" was just updated for
consistency, although it was not really needed.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>