Allow an empty value to be specified for jobs.<id>.container

[ericsciple]

Is your feature request related to a problem? Please describe.

I'd like to be able to build a matrix workflow where some jobs run in a container and some run directly on the VM. I may have a cross-platform tool where I have strange dependencies that I provide via a container, or I want to run on multiple different Linux distributions, but also run on Windows and macOS. So I would like a way to specify that I run on no container. This would allow me to create a workflow like:

name: CI

on: [push]

jobs:    
  build:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        image: ['ubuntu:latest', 'centos:latest', '']

A more realistic scenario would be a matrix of runs-on that includes linux, windows and macos, and then includeing the container value on linux hosts.

Describe the solution you'd like
That some value exists for container that means "none". Perhaps: an empty string (''), a dash ('-'), or a dot ('.'), all of which are not legal image names.

[ericsciple]

@ethomson fyi i moved the issue here

[ericsciple]

Fixed on the runner (expression scenario). Still need to fix on the server (non-expression)

[mnishamk-freiheit]

could this be done for jobs.<id>.container.image as well?

ie support the use of private images or no images