The following document: https://gist.github.com/gelim/a840a99d15cb765cb7b65105b72f00c4
is a guide to installing plain ShareLaTeX (without Docker). Basically, the author shows how to get the source for the Docker image and make the installation from it on a Ubuntu system. This would perhaps help a Debian packager. Note that this installation method is not supported by the upstream authors of ShareLaTeX (they only support installation of the Docker image provided by them). J.