You have kubernetes cluster running your application container and SQL Server which is running on server outside cluster. If you want to test connectivity to SQL Server from within container then you can install sqlcmd client utility.

Steps to install sqlcmd on Ubuntu 16.04:

Connect to the container shell using kubectl exec:

kubectl -n <namespace> exec -it <test-007> — /bin/bash

Run these commands-

sudo curl -o /etc/apt/sources.list.d/mssql-server.list https://packages.microsoft.com/config/ubuntu/16.04/mssql-server-2017.list

sudo curl https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -

sudo apt-get update

 
If it shows the below highlighted error, then install apt-transport-https

Reading package lists... Done
W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list.d/microsoft-prod.list:1 and /etc/apt/sources.list.d/msprod.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list.d/microsoft-prod.list:1 and /etc/apt/sources.list.d/msprod.list:1
E: The method driver /usr/lib/apt/methods/https could not be found.
N: Is the package apt-transport-https installed?
E: The method driver /usr/lib/apt/methods/https could not be found.
N: Is the package apt-transport-https installed?

E: Failed to fetch https://packages.microsoft.com/ubuntu/16.04/prod/dists/xenial/InRelease
E: Failed to fetch https://packages.microsoft.com/ubuntu/16.04/mssql-server-2017/dists/xenial/InRelease

E: Some index files failed to download. They have been ignored, or old ones used instead.
W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list.d/microsoft-prod.list:1 and /etc/apt/sources.list.d/msprod.list:1
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list.d/microsoft-prod.list:1 and /etc/apt/sources.list.d/msprod.list:1


sudo apt-get install apt-transport-https

sudo apt-get update

sudo apt-get  install mssql-tools

ls /opt/mssql-tools/bin/sqlcmd*

ln -sfn /opt/mssql-tools/bin/sqlcmd /usr/bin/sqlcmd

Once installed you can try sqlcmd connection to SQL Server

sqlcmd -S <sqlservername> -U <dbusername> -P <dbuserpassword> -d <database to connect>