migrate docker image to this repo

We used to develop the docker image in a separated repo: https://gitlab.torproject.org/tpo/anti-censorship/docker-snowflake-proxy/

But now we have a CI building the docker image in this repo: !246 (merged)

Let's deprecate the original docker repo and move everything here. Things that might be missing:

• move docker-compose.yml to this repo or somewhere
• update the community documentation to use our repo
• integrate publishing the docker image in the release process
• are we cross building in the CI?
• how are we going to push to dockerhub the image?
• archive docker-snowflake-proxy repo