Github actions: add and refer secrets

 Repository direct link:

https://github.com/<Org/Account>/<Repo>/settings/secrets/actions

Or open repo url --> settings tab -->secrets --> actions --> Repository/Environment/Organization secrets

Those you can refer using ${{secrets.<variablename>}}

- name: Setup ACR
	uses: azure/docker-login@v1
	with:
	login-server: ${{ env.container_registry }}
	username: ${{ secrets.ACR_USERNAME }}
	password: ${{ secrets.ACR_PASSWORD }}