Add multi-stage builds for Docker

_This issue was originally opened by @finferflu as hashicorp/packer#9462. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

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#### Feature Description

Add the ability to create [multi-stage builds](https://docs.docker.com/develop/develop-images/multistage-build/).

#### Use Case(s)

This is vital to keep the image sizes down, e.g. I have a Dockerfile which I can't translate to Packer:

```dockerfile
FROM prom/prometheus:v2.19.0

FROM alpine:3.10.2
RUN apk add gettext

COPY --from=0 /bin/prometheus /bin/prometheus

RUN mkdir -p /prometheus /etc/prometheus && \
chown -R nobody:nogroup etc/prometheus /prometheus
# Run envsubst before Prometheus.
RUN echo $'#!/bin/sh\n\
envsubst < /etc/prometheus/orig.yml > /etc/prometheus/prometheus.yml && \
exec /bin/prometheus "$@"' \
> /etc/prometheus/entrypoint.sh
RUN chmod +x /etc/prometheus/entrypoint.sh
ENTRYPOINT ["/etc/prometheus/entrypoint.sh"]

CMD [ "--config.file=/etc/prometheus/prometheus.yml", \
"--storage.tsdb.path=/prometheus" ]
USER nobody
EXPOSE 9090
VOLUME [ "/prometheus" ]
WORKDIR /prometheus

# This is your local prometheus.yml.
ADD prometheus.yml /etc/prometheus/orig.yml
```