Add docker-compose override file for Traffic Monitoring

[tstadel]

Sometimes it's invaluable to inspect and monitor the traffic between ui, rest_api and elasticsearch: E.g. ensuring request correlation, debugging elasticsearch problems, etc.). Till now there's no easy way to monitor traffic. A simple docker-compose override file makes it super easy for anyone without the need to fiddle with the own environment.

Proposed changes:

• add docker-compose override file which enables HTTP traffic monitoring between containers using mitmproxy
• add use_system_proxy param to ElasticSearchDocumentStore to force use of RequestsHttpConnection in elasticsearch client which automatically uses the system proxy (e.g. HTTP_PROXY and HTTPS_PROXY env variable in linux)

Usage:
Run

docker-compose -f docker-compose[-gpu].yml -f docker-compose.mitm.yml up

Watch traffic on localhost:8081.
(You might need to rebuild rest_api docker container, to enable use_system_proxy)

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code

[ZanSara]

Oh no, another parameter to the __init__ method of ElasticsearchDocumentStore!! 😄

Just kidding, it's the right thing to do here. However, __init__ parameters are really proliferating. I'll add this PR as yet another reason to review docstores' architecture.

[julian-risch]

LGTM! 👍