feat(postgres): add PostgreSQL 18 Alpine compose file

- Add docker-compose.yaml with POSTGRES_USER, POSTGRES_PASSWORD, and
  POSTGRES_DB environment variables
- Add .env.example with documented defaults
- Remove Contents section from README

compose/postgres/.env.example

@@ -0,0 +1,13 @@
+# PostgreSQL (Alpine) — environment variables
+# Copy this file to .env and adjust values for your environment.
+
+# Superuser name (default: postgres).
+POSTGRES_USER=postgres
+
+# Superuser password.
+# The default is for local development only — always change for production.
+POSTGRES_PASSWORD=postgres
+
+# Default database created automatically on first start.
+# Defaults to the value of POSTGRES_USER if not set.
+POSTGRES_DB=postgres