GitHub workflow example using env variables and a repository dispatch trigger for the workflow

var_and_trigger.yml

# This workflow can be triggered w/ a dispatch event, e.g.:
#  curl -X POST -H "Authorization: token $(cat /tmp/token_file)" \
#    -H "Content-Type: application/json" -H "Accept: application/vnd.github.v3+json" \
#    https://api.github.com/repos/{owner}/{repo}/dispatches \
#    -d '{"event_type":"test-trigger", "client_payload": {"label": "latest", "date_tag":true}}'
# A valid GitHub token must be saved in the file (so that is presented in the authorization)
# otherwise a misleading "Not Found" message is returned. No reply is provided for successful posts.
# The client_payload label and date_tag are optional
#  label is used as Docker Hub label instead of "latest"
#  if date_tag id true a date tag is added to the Docker Hub label (+%Y%m%d-%H%M)
# -X POST is also optional

name: Variable test

on:
  push:
    branches: [ mybranch ]
  repository_dispatch:
    types:
      - test-trigger

env:
  MY_LABEL: latest

jobs:
  testvar:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2

    - name: change1
      run: |
        echo "MY_LABEL=changed" >> $GITHUB_ENV

    - name: change2
      continue-on-error: true
      if: ${{ github.event.client_payload.label }}
      run: |
        echo "MY_LABEL=${{ github.event.client_payload.label }}" >> $GITHUB_ENV

    - name: make date tag
      id: mkdatetag
      #if: success() || failure() # run regardless of prior step success/failure
      run: echo "::set-output name=dtag::$(date +%Y%m%d-%H%M)"

    - name: add date tag
      continue-on-error: true
      if: ${{ github.event.client_payload.date_tag }}
      run: |
        echo "MY_LABEL=$MY_LABEL-${{ steps.mkdatetag.outputs.dtag }}" >> $GITHUB_ENV

- name: echos
      run: |
        echo "<MY_LABEL is $MY_LABEL, label is ${{ github.event.client_payload.label }}, dtag is ${{ steps.mkdatetag.outputs.dtag }}>"