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

README.md

@@ -3,13 +3,6 @@
 A collection of Docker infrastructure files for quick deployment of
 development services.
 
-## Contents
-
-- **`compose/`** — Docker Compose files for various services
-  - [`sql-server-2025`](compose/sql-server-2025/) — Microsoft SQL Server 2025
-- **`get-docker.sh`** — Docker Engine installation script for Linux, synchronised
-  weekly from upstream
-
 ## Usage
 
 Each service lives under its own directory within `compose/`. Copy the

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

compose/postgres/docker-compose.yaml

@@ -0,0 +1,33 @@
+# Docker Compose file for PostgreSQL (Alpine)
+#
+# Usage:
+#   docker compose up -d
+#
+# Environment variables (see .env.example for details):
+#   POSTGRES_USER     - Superuser name (default: postgres)
+#   POSTGRES_PASSWORD - Superuser password (default: postgres)
+#   POSTGRES_DB       - Default database created on first start (default: postgres)
+
+services:
+  postgres:
+    # For available image tags, see https://hub.docker.com/_/postgres
+    image: postgres:18-alpine
+    container_name: postgres
+    environment:
+      # Superuser name. Defaults to "postgres" if not set.
+      - POSTGRES_USER=${POSTGRES_USER:-postgres}
+      # Superuser password.
+      # Default value is for local development only — change for production.
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-postgres}
+      # Database created automatically on first start.
+      # Defaults to the value of POSTGRES_USER if not set.
+      - POSTGRES_DB=${POSTGRES_DB:-postgres}
+    ports:
+      # Map host port 5432 to container port 5432 (PostgreSQL default).
+      - "5432:5432"
+    volumes:
+      # Persist data beyond container lifetime.
+      - postgres_data:/var/lib/postgresql/data
+
+volumes:
+  postgres_data: