Container doesn't build anymore

It looks like there's been a new Debian release that no longer includes the libc-client-dev package, which is needed for IMAP support in PHP. It looks like the base image now pulls in that latest version, and then the build crashes. Upstream has fixed it by specifying the older Debian explicitly: https://github.com/martialblog/docker-limesurvey/commit/ed3f92cf9bb1a1df13e7399b01ac12b022a787bb