Generated Dockerfile always uses default image name

I am using `21-temurin`, not `21-temurin-alpine` (the default), but the generated Dockerfile uses the latter for its name:

<img width="384" alt="Image" src="https://github.com/user-attachments/assets/14b0c4e8-9f8d-44c0-9d90-10d8f250c44b" />