Add action for building other docker images, add a test for open-iscsi #870

	name: Build docker image
	on:
	push:
	branches:
	- 'flatcar-master'
	pull_request:
	branches:
	- 'flatcar-master'
	env:
	REGISTRY: ghcr.io
	IMAGE_NAME: flatcar/mantle

	jobs:
	docker:
	runs-on: ubuntu-latest
	permissions:
	contents: read
	packages: write
	steps:

	- name: Get tag name
	id: meta
	uses: docker/metadata-action@v5
	with:
	images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
	tags: \|
	type=sha,event=branch,enable=${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) }},prefix=git-,suffix=,format=long
	type=ref,event=pr
	type=ref,event=tag

	- name: Checkout repository
	uses: actions/checkout@v4
	with:
	fetch-depth: 0

	- name: Setup QEMU
	uses: docker/setup-qemu-action@v3
	with:
	platforms: all

	- name: Set up Docker Buildx
	uses: docker/setup-buildx-action@v3

	- name: Login to registry
	uses: docker/login-action@v3
	with:
	registry: ${{ env.REGISTRY }}
	username: ${{ github.actor }}
	password: ${{ secrets.GITHUB_TOKEN }}

	- name: Build and push
	uses: docker/build-push-action@v5
	with:
	context: .
	push: true
	platforms: linux/amd64,linux/arm64/v8
	tags: ${{ steps.meta.outputs.tags }}
	labels: ${{ steps.meta.outputs.labels }}
	cache-from: type=gha
	cache-to: type=gha,mode=max

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