Extract tag in github actions

demo.yaml

# !! restriction: trigger must be push:tags !!

name: Demo
on:
  push:
    tags:
      - v[0-9]+*

permissions:
  contents: read
  packages: write

env:
  REGISTRY: ghcr.io
  IMAGE_NAME: ${{ github.repository_owner }}/demo

jobs:

  release:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      # key step
      # after this, you can use ${{ steps.tag_out.outputs.tag }}
      # to read tag value
      - name: Set tag output
        id: tag_out
        run: echo ::set-output name=tag::${GITHUB_REF#refs/*/}

      - name: Docker Login
        uses: docker/login-action@v2.0.0
        with:
          registry: ${{ env.REGISTRY }}
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}
      - name: Build Docker Image
        run: docker build .
      - name: Push Docker
        env:
          RELEASE_VERSION: ${{ steps.tag_out.outputs.tag }}
        run: |
          docker tag ramnew/bpu:1 $REGISTRY/$IMAGE_NAME:${RELEASE_VERSION/v/}
          docker tag ramnew/bpu:1 $REGISTRY/$IMAGE_NAME:latest
          docker push $REGISTRY/$IMAGE_NAME:${RELEASE_VERSION/v/}
          docker push $REGISTRY/$IMAGE_NAME:latest