Remove hard-coded credentials from docker-compose.yml

[Copilot]

docker-compose.yml shipped hard-coded PostgreSQL, service, and pgAdmin credentials. These are now supplied via environment variables loaded from a git-ignored .env file.

Changes

• docker-compose.yml — Replaced literal credentials with ${VAR} substitutions across all three services:
  • Connection string → ${POSTGRESQL_USERNAME} / ${POSTGRESQL_PASSWORD} / ${POSTGRESQL_DATABASE}
  • ServicePassword → ${SERVICE_PASSWORD}
  • Postgres POSTGRES_USER/PASSWORD/DB, pgAdmin PGADMIN_DEFAULT_EMAIL/PASSWORD, and the pgpassfile entrypoint line
• .env.example — New template enumerating the required variables; developers copy it to .env.
• .gitignore — Ignore .env so real credentials stay out of source control.
• README.md — Development setup now includes the copy-.env.example-to-.env step.

environment:
    ConnectionStrings__CloudLibraryPostgreSQL: "Server=db;Username=${POSTGRESQL_USERNAME};******;Database=${POSTGRESQL_DATABASE};Port=5432;Ssl Mode=Prefer"
    ServicePassword: ${SERVICE_PASSWORD}

Notes

• .env.example retains the prior uacloudlib defaults purely as placeholder values for local dev convenience; the actual .env is untracked.
• docker compose config resolves all variables from .env as expected.

• Fixes Remove hard-coded creds from docker_compose.yaml #288