Working in a programming company whose clients include riba-based banks and companies that produce alcohol
It is not permissible to offer assistance or services or programming to riba-based banks or companies that produce alcohol, because that is helping in sin and transgression, and Allah says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
The one who helps another to do evil or shows him the way to it is a partner in the sin and has failed to do what Allah has enjoined of denouncing and disavowing evil.
Thus you know that it is not permissible to contribute to maintenance or programming that has to do with these haraam companies, whether that is done regularly or rarely. If you know that a client has a company that does haraam things, it is not permissible to you to provide services to him.
There is nothing wrong with you remaining in your job if you can avoid participating in haraam work. But if you can find another job that is free of the things we have mentioned, then that is better.
And Allah knows best.