Composite action

[crazy-max]

Fixes #208

Try-out the composite feature which now supports uses: keyword:

• Set up Buildx and QEMU
• Cache
• Metadata
• Build and push

Usage:

name: ci

on:
  push:
    branches:
      - 'master'

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      -
        name: Checkout
        uses: actions/checkout@v2
      -
        name: Login to DockerHub
        uses: docker/login-action@v1
        with:
          username: ${{ secrets.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
      -
        name: Build with composite
        uses: docker/build-push-action/composite@v2
        with:
          context: .
          images: |
            user/app
          platforms: linux/amd64,linux/arm64
          push: ${{ github.event_name != 'pull_request' }}

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[crazy-max]

We would prefer reusable workflows for such case, see #1321