Clarify how to merge in ReleaseGuide.md #62

	name: Update docker image

	on:
	workflow_dispatch: # Trigger by hand from the UI
	inputs:
	branch:
	type: choice
	description: branch to build the container from
	options:
	- develop
	push:
	branches:
	- develop

	jobs:
	build-and-release-docker-image:
	name: Builds a dockerimage with the fenics adapter of preCICE
	runs-on: ubuntu-latest
	env:
	docker_username: precice
	steps:
	- name: Set branch name for manual triggering
	if: github.event_name == 'workflow_dispatch'
	shell: bash
	run: \|
	echo "ADAPTER_REF=${{ inputs.branch }}" >> $GITHUB_ENV
	- name: Set branch name for on pull triggering
	if: github.event_name != 'pull_request' && github.event_name != 'workflow_dispatch'
	shell: bash
	run: \|
	echo "ADAPTER_REF=${{ github.ref_name }}" >> $GITHUB_ENV
	- name: Set PYTHON_BINDINGS_REF and the TAG depending on branch
	shell: bash
	run: \|
	echo "PYTHON_BINDINGS_REF=${{ env.ADAPTER_REF }}" >> "$GITHUB_ENV"
	echo "TAG=${{ env.ADAPTER_REF }}" >> "$GITHUB_ENV"
	echo "Building TAG: ${{ env.ADAPTER_REF }}"
	- name: Checkout Repository
	uses: actions/checkout@v2
	- name: Set up Docker Buildx
	uses: docker/setup-buildx-action@v1
	- name: Login to DockerHub
	uses: docker/login-action@v1
	with:
	username: ${{ env.docker_username }}
	password: ${{ secrets.DOCKERHUB_TOKEN }}
	- name: Build and push Dockerfile
	uses: docker/build-push-action@v2
	with:
	push: true
	file: "./tools/releasing/packaging/docker/Dockerfile"
	tags: ${{ env.docker_username }}/fenics-adapter:${{ env.TAG }}
	build-args: \|
	FENICS_ADAPTER_REF=${{ env.ADAPTER_REF }}
	PYTHON_BINDINGS_REF=${{ env.PYTHON_BINDINGS_REF }}

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