Can we define which k8s manifests epinio should deploy when a new application is installed ?

Since version 0.8.0 (today's release), you can configure the application deployment by creating a custom application helm chart. Documentation is here: https://docs.epinio.io/howtos/create_custom_appcharts It's the first release which includes this feature so there are probably things to improve. Let us know how it works for you and feel free to open issues for bugs or suggestions

Do you know that if the developers use dekorate.io, then we can (without the need to write the k8s json or yaml files which is really a pain) generate the k8s manifests or the Helm chart ?

I haven't used dekorate (not a Java dev here), but afaict dekorate is run as part of the build process of the app and creates kubernetes yaml that can be used to deploy the app. Epinio expects the outcome of the build process to be a container image, not the helm chart itself (or resource yamls). I wonder if it would be possible to let dekorate.io create those yamls in a pipeline somewhere (outside Epinio) and have that pipeline build a helm chart as well. That pipeline could then update the application helm chart epinio uses behind the scenes. It's hard to come up with a workflow without an actual example of an application though. I could be completely off with my suggestion.