Support multiple platforms for the Docker images

[adamtheturtle]

If we follow the instructions in https://vws-python-mock.readthedocs.io/en/latest/docker.html, we get warnings like:

WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested

This issue covers removing that.

I believe that the steps include:

• Use Buildx
• Set multiple platforms

Buildx support may start with:

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v1

We may run into docker/buildx#593.

[adamtheturtle]

We are now using Buildx but with the docker driver (not docker-container).
This doesn't allow for multiple platforms.
Next step is likely to switch to the docker-container driver and then use:

platforms: linux/amd64,linux/arm64

where we want.