Add warning when Docker installation not found

[nelsonni]

Version: v0.2.17
Platform: MacOS
Command: act -n
main.workflow file:

name: Manually triggered workflow
on:
  workflow_dispatch:
    inputs:
      name:
        description: 'Person to greet'
        required: true
        default: 'Mona the Octocat'
      home:
        description: 'location'
        required: false
        default: 'The Octoverse'

jobs:
  say_hello:
    runs-on: ubuntu-latest
    steps:
    - run: |
        echo "Hello ${{ github.event.inputs.name }}!"
        echo "- in ${{ github.event.inputs.home }}!"

Output:

(blank)

Expected Output:
A warning indicating that Docker needs to be installed.

If Docker is installed, but the Docker Daemon isn't running, then there is the following warning:

Error: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Can this type of check and warning be replicated when Docker is missing?

[nelsonni]

This is a partial continuation of the request for further documentation to indicate that Docker is a prerequisite for using act:

Make sure docker is running 😉

I think the documentation could have a requirements, as it states that it uses the Docker API, but it does not specify that you need an installation of Docker.

Originally posted by @GustavoFenilli in #176 (comment)

[github-actions]

Issue is stale and will be closed in 14 days unless there is new activity

[nelsonni]

This issue is not stale. PR #367 discusses a solution that would partially address this concern, but in case a user doesn't realize that Docker is a required dependency a runtime warning would be helpful.