Provide updated Mac install instructions

[dtrudg]

Singularity Desktop for Mac is no longer updated. Provide instructions for vagrant on Mac.

[MaxCan-Code]

How about instructions for Docker on Mac/Windows? I have a working compose.yaml using the latest image on quay.io

services:
  singularity:
    image: quay.io/singularity/singularity:v3.7.4-slim 
    stdin_open: true
    tty: true
    privileged: true
    volumes:
      - .:/root
    entrypoint: ["/bin/sh"]

, which is a slimmed down verson of https://github.com/horothesun/singularity-conda-multiple-envs#-local-singularity-image-build. I can add the instructions if you'd like

[dtrudg]

Singularity in Docker can have various disadvantages, such that not all features describe in the guides may work as expected. Basic container operations will work, but Singularity has a number of other lesser used features.

It's a nice idea to add the instructions, but we'd have to clearly describe the issues. Historically we have avoided inferring support for nesting Singularity in Docker, so it needs a bit of discussion around whether that's something we want to do.

[MaxCan-Code]

I see, is this enough of a discussion?

--------------------------        
{Singularity} Docker Image
--------------------------

It is possible to use a Dockerized Singularity,
here is a sample ``compose.yaml`` (Singularity version 3.7.4) for use with Docker Compose:

.. code-block:: none

    services:
      singularity:
        image: quay.io/singularity/singularity:v3.7.4-slim 
        stdin_open: true
        tty: true
        privileged: true
        volumes:
          - .:/root
        entrypoint: ["/bin/sh"]

Singularity in Docker can have various disadvantages,
but basic container operations will work.
Currently, the intended use case is continuous integration,
meaning that you should be able to build a Singularity container using this Docker Compose file.
For more information see `issue#5 <https://github.com/sylabs/singularity-admindocs/issues/5#issuecomment-852307931>`_
and the image's source `repo <https://github.com/singularityhub/singularity-docker#use-cases>`_