fix: clean up Docker setup (compose, README, gitignore)

- Remove deprecated version field from docker-compose.yml
- Pin Postgres to 18-alpine (matches existing db-data)
- Remove undocumented web (Apache) service
- Fix config/bin volume mounts to use docker/ directory
- Gitignore docker/savedata, docker/bin, docker/config.json
- Rewrite docker/README.md: fix typos, use docker compose V2
  commands, match actual compose file behavior
- Link docker/README.md from main README Docker section

.gitignore

@@ -8,6 +8,9 @@ savedata/*/
 *.lnk
 *.bat
 /docker/db-data
+/docker/savedata
+/docker/bin
+/docker/config.json
 screenshots/*
 
 # We don't need built files