Optimized Dockerfile template for building custom Prompt Flow containers.

Is your feature request related to a problem? Please describe.
The feature request is not related to a problem, but a possibility to optimize Prompt Flow performance.

Describe the solution you'd like
The pf flow build --format docker command adds a Dockerfile based on docker.io/continuumio/miniconda3:latest as can be found in:

https://github.com/microsoft/promptflow/blob/8e55822664ed70b37074fbd77b11a8351c5e0ece/src/promptflow/tests/test_configs/flows/export/linux/Dockerfile#L2

This works, but results in long Docker image build times and possibly unwanted overhead.

Describe alternatives you've considered
I found that substituting this with the following Dockerfile results in much faster build times and much smaller and faster pushes to a container registry:

# syntax=docker/dockerfile:1
FROM mcr.microsoft.com/azureml/promptflow/promptflow-runtime-stable:latest

WORKDIR /

COPY ./flow /flow

EXPOSE 8080

COPY ./connections/* /connections/

RUN pip install keyrings.alt -r /flow/requirements.txt

# reset runsvdir
RUN rm -rf /var/runit
COPY ./runit /var/runit
# grant permission
RUN chmod -R +x /var/runit

COPY ./start.sh /
CMD ["bash", "./start.sh"]

Additional context
This documentation page also mentiones, that Dockerfiles created to run Prompt Flow should be based on promptflow-runtime-stable.