✨ Adds project guidelines and conventions for Github Copilot

Introduces detailed instructions for ASP.NET API, C#, Docker, MSBuild,
testing, and library creation. These guidelines aim to standardize
development practices and improve code quality across the project.

.github/instructions/docker.instructions.md

@@ -0,0 +1,31 @@
+---
+description: "Use when editing Dockerfile, compose files, or Docker-related configuration. Covers guardrails for the multi-stage build and runtime image selection."
+applyTo: "docker/**,compose*.yaml"
+---
+# Docker Conventions
+
+`docker/Dockerfile` is a multi-stage pipeline — stage names and purposes are visible in the file itself.
+
+## Guardrails
+
+- **Build context** is always the repo root, not `docker/`.
+- **`COPY --link`** is used intentionally for layer cache — do not replace with plain `COPY`.
+- **Cache mounts** (`--mount=type=cache,target=/root/.nuget/packages`) must stay on restore, build, test, and publish steps.
+- **Stage dependency chain** must be preserved (e.g., `build` → `restore`, `test` → `build`, `publish` → `test`).
+- New stages should follow the existing pattern: `FROM <parent> AS <name>`.
+
+## Build Args
+
+- `RUNTIME_BASE`: `web` (ASP.NET), `app` (console), or `self-contained`
+- `CONFIGURATION`: `Release` (default) or `Debug`
+- `PUBLISHED_PROJECT`: Project name to publish (without path or extension)
+- `DATABASE_PROJECT`: EF Core project for migration bundles
+- `DBCONTEXT`: Optional DbContext name for migrations
+
+## Image Selection
+
+| Use Case | `RUNTIME_BASE` | Base Image |
+|----------|----------------|------------|
+| ASP.NET web apps | `web` | `aspnet:*-alpine-composite-extra` |
+| Console / worker | `app` | `runtime:*-alpine-extra` |
+| Self-contained | `self-contained` | `runtime-deps:*-alpine-extra` |