Add singularity support

[IllustratedMan-code]

It would be amazing if there could be a mkSingularityImage function for singularity containers.

As far as I can tell, implementation will be very similar to the existing mkDockerImage function that already exists. There is a very similar nix package to dockerTools called singularity-tools in the nixpkgs repository.

the buildImage function in singularity-tools is used in a very similar way to the existing implementation here

[ShamrockLee]

The singularity-tool is currently under refactoring (NixOS/nixpkgs#158486 , NixOS/nixpkgs#177908). A Singularity / Apptainer definition file generator and a definition-based buildImage would be added in a future PR (NixOS/nixpkgs#158486 (comment) , NixOS/nixpkgs#158486 (comment)) if things work out as expected.

We could also try to think of a way to generate a Singularity / Apptainer definition.