update actions/cache to v4 due to EOL #1046

	name: Push to GCR GitHub Action
	on:
	push:
	branches:
	- '**'
	tags:
	- '**'
	# pull_request:
	# branches:
	# - 'main'
	jobs:
	check-env:
	runs-on: ubuntu-latest
	# assign output from step to job output
	outputs:
	gcloud-service-key: ${{ steps.gcloud-service-key.outputs.defined }}
	steps:
	- id: gcloud-service-key
	# assign GCLOUD_SERVICE_KEY to env for access in conditional
	env:
	GCLOUD_SERVICE_KEY: ${{ secrets.GCLOUD_SERVICE_KEY }}
	if: "${{ env.GCLOUD_SERVICE_KEY != '' }}"
	# runs if GCLOUD_SERVICE_KEY is defined, so we set the output to true
	run: echo "::set-output name=defined::true"

	build-and-push-to-gcr:
	runs-on: ubuntu-latest

	# uses check-env to determine if secrets.GCLOUD_SERVICE_KEY is defined
	needs: [check-env]
	if: needs.check-env.outputs.gcloud-service-key == 'true'

	steps:
	- uses: actions/checkout@v2
	-
	name: Docker meta
	id: meta
	uses: docker/metadata-action@v3
	with:
	# list of Docker images to use as base name for tags
	images: \|
	gcr.io/clabs-optics/optics-agent
	# generate Docker tags based on the following events/attributes
	tags: \|
	type=ref,event=branch
	type=ref,event=pr
	type=sha
	-
	name: Login to GCR
	uses: docker/login-action@v1
	with:
	registry: gcr.io
	username: _json_key
	password: ${{ secrets.GCLOUD_SERVICE_KEY }}
	-
	name: Build and push
	uses: docker/build-push-action@v2
	env:
	DOCKER_BUILDKIT: 1
	with:
	context: ./rust
	file: ./rust/Dockerfile
	push: true
	tags: ${{ steps.meta.outputs.tags }}
	labels: ${{ steps.meta.outputs.labels }}

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