Enable multi-platform build

[christian-kreuzberger-dtx]

If we ever want to build for raspberry PI or any other non amd64 based platform, we should enable buildx.

One way to achieve this would be by using the following workflow:

        name: Set up QEMU
        uses: docker/setup-qemu-action@v1
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v1
      -
        name: Login to DockerHub
        uses: docker/login-action@v1 
        with:
          username: ${{ secrets.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
      -
        name: Build and push
        uses: docker/build-push-action@v2
        with:
          context: .
          file: ./Dockerfile
          platforms: linux/386,linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/ppc64le,linux/s390x
          push: true
          tags: |
            user/app:latest
            user/app:1.0.0

Alternatively, instead of using the docker/build-push-action, we can do the same via docker buildx build --platform ....

[christian-kreuzberger-dtx]

FYI, for Keptn we should try to reduce the number of platforms to a bare minimum, e.g.:
linux/amd64 + linux/arm64 (and maybe linux/arm/v7 for raspberry pi 3)

[christian-kreuzberger-dtx]

Tried it... was super slow (more than 5 times slower by just adding arm64 and arm/v7). I'm putting this on hold for now.