Reduce unused services in Docker Compose

[dhruvkb]

Problem

The following services in Docker Compose are largely unused.

• load_to_s3

  I could not find any occurrences of this string in the project except for the docker-compose.yml file. This leads me to suspect it might actually be completely unused.

• nginx

  This runs the nginx target from the API Dockerfile. It's purpose in the local dev environment is unclear and building it when initialising the API slows down CI.

• proxy

  proxy was added long back to test for HTTPS with the API server during local development. It also introduces a dependency on mkcert so removing it will have twice the benefit.

They should be removed or, if they are in fact used, comments should be added to list down what they are used for and where.

Additional context

This is part of a bigger improvement of the Docker Compose configuration that involves upgrading to v3 #4046 and is closely linked to #1910. This was also raised by @krysal in #1009.