Fix Docker build errors when opening a PR from a fork.

[antoinevg]

As per spec :-)

• Main check is:

  packetry/.github/workflows/appimage.yml

  Lines 37 to 42 in de35e6b

  	- name: Use 'main' Docker image tag if this is a PR from a fork
  	id: origin
  	shell: bash
  	if: github.event.pull_request.head.repo.fork
  	run: |
  	echo 'IMAGE_TAG=main' >> $GITHUB_ENV

• Because the logic for whether we're doing a build or not is getting complex it now gets its own step:

  packetry/.github/workflows/appimage.yml

  Lines 67 to 73 in de35e6b

  	- name: Check if we need to build the Docker image
  	id: dockerimage
  	if: |
  	! github.event.pull_request.head.repo.fork &&
  	(steps.changed.outputs.dockerfile == 'true' || steps.exists.outputs.dockerimage == 'false')
  	run: |
  	echo 'build=true' >> $GITHUB_OUTPUT

Only other change is a smol output of the Docker name & tag used for any given build to avoid any potential confusion:

packetry/.github/workflows/appimage.yml

Lines 124 to 127 in de35e6b

	- name: Check image name & tag
	run: |
	echo "NAME: ${{ needs.create-docker-image.outputs.NAME }}"
	echo "TAG: ${{ needs.create-docker-image.outputs.TAG }}"

Also been checked as an upstream PR just to make sure it didn't break anything: #151