Add multi_dockerignore option

[karaxuna]

Change-type: minor

[karaxuna]

@thgreasi can I get a review?

[thgreasi]

@karaxuna after enabling external contributors, I now see a different error in CI
See: https://github.com/balena-io/deploy-to-balena-action/actions/runs/4505762867/jobs/7932156158#step:4:95

Can you try removing this ' and see whether that fixes the yml?

[klutchell]

The issue here is that this secondary job

deploy-to-balena-action/.github/workflows/flowzone.yml

Lines 30 to 51 in 72464a1

	test-deploy-to-cloud:
	name: Test deploy to balenaCloud
	runs-on: ubuntu-latest
	needs: flowzone
	if: github.event.action != 'closed'
	steps:
	- uses: actions/checkout@v3
	- name: Replace the build image and Dockerfile for testing
	run: |
	# Use the Docker image built from the PR
	yq e -i '.runs.image = "docker://ghcr.io/balena-io/deploy-to-balena-action:${{ github.event.pull_request.head.sha }}"' \
	action.yml
	# Overwrite the Dockerfile with minimal number of steps so the test deploy finishes quickly
	echo FROM busybox > Dockerfile
	- name: Deploy to a test fleet
	uses: ./
	with:
	balena_token: ${{ secrets.BALENA_API_KEY }}
	fleet: balena/deploy_to_balena_action
	cache: false

Was not designed with pull_request_target (external contributions) in mind. We should change that job to a Flowzone custom test composite action and perform the same steps, but without the checkout.

[thgreasi]

Thanks for the PR @karaxuna and sorry for the delay on reviewing (and extending flowzone to allow external contributor PRs).