This is a simple image template that can get you started in a Ruby container with FreeTDS installed so that you can use the TinyTDS gem to talk to MS SQL Server.
It starts with a base Ruby image, and downloads and builds FreeTDS from source.
Grab the Dockerfile expand upon it, or build and use it as a base image.
docker build -t ruby-freetds .
You can also specify the version of FreeTDS that gets installed with an arg:
docker build -t ruby-freetds --build-arg FREETDS=freetds-1.00.95 .
By default it uses 1.00.85 which has been tested to work correctly with at least tiny_tds (2.1.1) and Ruby (2.4).
The FREETDS var is used to interpolate the following url to download from:
wget http://www.freetds.org/files/stable/$FREETDS.tar.gz
So, make sure that whatever you use actually exists.