Simplify envvar handling logic #9

	---

	# Derivative of the following pipeline by Lars Kellogg-Stedman
	# https://github.com/larsks/hello-flask
	# GPLv3

	name: 'Build and publish container images'

	on:
	push:
	branches:
	- tes3mp-*

	jobs:
	docker:
	runs-on: ubuntu-20.04
	steps:

	- name: Checkout
	uses: actions/checkout@v2

	- name: Prepare
	id: prep
	run: \|
	DOCKER_IMAGE=tes3mp/server
	VERSION=$(grep "ENV TES3MP_VERSION .*" Dockerfile \| awk 'NF>1{print $NF}')
	TAGS="${DOCKER_IMAGE}:${VERSION}"
	if [[ $GITHUB_REF =~ master ]]; then
	TAGS="$TAGS,${DOCKER_IMAGE}:latest"
	fi
	echo ::set-output name=tags::${TAGS}
	echo ::set-output name=docker_image::${DOCKER_IMAGE}

	- name: Set up QEMU
	uses: docker/setup-qemu-action@master
	with:
	platforms: all

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

	- name: Login to DockerHub
	if: github.event_name != 'pull_request'
	uses: docker/login-action@v1
	with:
	username: ${{ secrets.DOCKER_USERNAME }}
	password: ${{ secrets.DOCKER_PASSWORD }}

	- name: Build
	uses: docker/build-push-action@v2
	with:
	builder: ${{ steps.buildx.outputs.name }}
	context: .
	file: ./Dockerfile
	platforms: linux/amd64,linux/arm
	push: true
	tags: ${{ steps.prep.outputs.tags }}

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