docs: add README, CONTRIBUTING, LICENCE, and .gitignore

- Add README with repository overview and usage instructions
- Add CONTRIBUTING with branch naming, commit, and PR guidelines
- Add MIT LICENCE
- Add .gitignore for Windows, macOS, Linux, IDE, and Claude Code files

README.md

@@ -0,0 +1,29 @@
+# docker-infra
+
+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
+`.env.example` to `.env`, adjust the values, then run:
+
+```sh
+cd compose/<service>
+docker compose up -d
+```
+
+## Licence
+
+This project is licensed under the MIT Licence — see [LICENCE](LICENCE) for
+details.
+
+Third-party software included in this repository (such as `get-docker.sh`)
+remains under its original licence terms.