Building custom containers

Currently the workflow for adding model (version)s to the eWaterCycle platform is (also see https://github.com/eWaterCycle/ewatercycle/issues/294)

1. make a dockerfile
2. build docker image
3. upload to dockerhub
4. build singularity pulling from dockerhub
5. use it

Step 3 and 4 can be combined by [building from a local docker image](https://github.com/apptainer/singularity/issues/1537), but it would be even better if we didn't need to convert between docker and singularity altogether.

One way to achieve this is to make docker available on the default infra
Another option is to switch from dockerfiles to something singularity understands (singularity definition file?)