Buster support

See merge request !62

update the README

Change the default email sender

See merge request !61

When called from a proxy, the X_API_ROOT_PATH used as entry point in the
proxy should not influence the call made to other parts of the API

As implemented for users-api-ror

with .deb version

Attempt to save the situation by reading GITLAB environment variables

These where detected by github

The issue has been seen with thin 1.7 in users-api-ror, but to keep both
application as close as possible as far as packaging and installation is
concerned, change this here

They cannot be avoided easily because bundler will change Gemfile.lock
to include version used to build bundle, and this will change between
debian versions

Packaging attempts to make sure git state is clean before packaging. If
it is not, output a more helpfull message