MSSQL support is missing

[nmusatov]

Currently image allows usage mysql, postgresql and sqlite, but doesn't allow to use mssql (which is also supported by redmine ootb).
Please add REDMINE_DB_MSSQL env configuration variable and underlying logic to use it with mssql ootb.

[nmusatov]

REDMINE_DB_SQLSERVER exists.
This means that documentation should be updated

[wglambert]

Ah yeah looks like it was implemented January 2018 #100