Should I be credited for my programming contribution to a dissertation?

You did work for hire for a research group. Since you say you were strictly involved in developing the tool and not in the research, authorship of subsequent publications is not to be expected. An acknowledgment in the thesis would be appropriate but it would have very little weight in a CV, inside or outside academia.

At any rate, you can definitely list your contribution to this project in your CV regardless of which kind of acknowledgment you might get. In fact having delivered a working software and gotten paid for it will be much more helpful than any kind of authorship/acknowledgments when applying for jobs outside academia.

I'm in a similar situation to yours, my job is a research support position, and it sounds like you're in a similar position. As in your situation, most of the work I do is on the technical side of things, supporting data collection for non-programming related research.

Whether or not you deserve credit for your work very much depends upon what you mean by credit. If you mean authorship-credit, I would probably say it's unlikely, unless the dissertation is going to discuss the use of your software as a novel research method. On the other hand, if you mean credit as in taking credit for designing and implementing the software, yes, you absolutely deserve credit, and you should be able to discuss your work as a project that you've worked on/are working on.

If the PhD candidate is gracious, they will give you an acknowledgement in their dissertation, however regardless of whether you are acknowledged, this is a project that you worked on, and should be able to discuss and claim as such.