Github action looping example. This is the only way to natively loop a step in GHA

GHA-looping.yml

name: listfiles

on:
  workflow_dispatch:

jobs:
  list-png-files:
    runs-on: ubuntu-latest
    outputs:
      file: ${{ steps.set-files.outputs.file }}
    steps:
      - uses: actions/checkout@v2
      - id: set-files
        run: echo "::set-output name=file::$(ls *.png | jq -R -s -c 'split("\n")[:-1]')"
  check:
    needs: list-png-files
    runs-on: ubuntu-latest
    strategy:
        matrix:
            file: ${{ fromJson(needs.list-png-files.outputs.file) }}
    steps:
      - run: |
          echo "${{ matrix.file }}"