use a more precise version of docker image, to lock ocaml version used and avoid installation problem for ocsigenserver.2.9

Revert "see if this makes a test fail"

This reverts commit f0f3a02f.

There was a discrepancy between the server and the client...

Notable change: we cannot attach to services with suffix parameters
any more; many /elections/$UUID/foo become /election/foo?uuid=$UUID.

Instead, we use a subdirectory "ballots/" of the election directory.