Docker endpoint, docs and env example

[chetanr25]

docker/entrypoint.sh + docker/.env.example + docker/README.md

What

• entrypoint.sh - shared init script for dev and prod containers: creates /data/db and /data/uploads if missing, runs DB init, then exec "$@" into the server (PID 1, receives signals correctly)
• .env.example - single template for all env vars; copy to .env.dev or .env.prod before first run
• docker/README.md - documents directory layout, volumes, env var notes, and migration guide from old root-level docker-compose.yml

Why

entrypoint.sh replaces inline shell in each compose command: field, so DB init logic lives in one place instead of being duplicated across dev and prod.

.env.example gives a single source of truth for required vars with inline comments.

Fixes: #516
Part of #512