Fix Postgres user name in docker-compose.yaml

[regnull]

Summary

Connection errors were generated when I ran docker compose locally (cannot connect as user postgres)

The Postgres docker doc clearly states that the user name should be POSTGRES_USER:

https://hub.docker.com/_/postgres

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[yihong0618]

did it maybe break old user's database?

[bowenliang123]

did it maybe break old user's database?

I don't think so. There's concret evidence that POSTGRES_USER is being used for long in PG 15 docker image for long.
see: https://github.com/docker-library/postgres/blob/master/15/bookworm/docker-entrypoint.sh#L92

[kurokobo]

Related to: #9888 (comment)

[laipz8200]

# ==================================================================
# WARNING: This file is auto-generated by generate_docker_compose
# Do not modify this file directly. Instead, update the .env.example
# or docker-compose-template.yaml and regenerate this file.
# ==================================================================