Add network & containerisation

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+# Docker – Basics
+
+Docker-specific concepts: architecture, images, containers, volumes, networks, and Compose.
+
+For generic containerisation concepts, see:
+
+- [Containerisation / Basics](../Basics.md)
+
+---
+
+## 1. What Is Docker?
+
+Docker is a platform built around Linux containers. It provides:
+
+- A **container runtime** (`dockerd`, the Docker daemon).
+- A **CLI** (`docker`) for building images, running containers, and managing the system.
+- A **build system** based on `Dockerfile`.
+- **Docker Compose** for defining and running multi-container applications from a single file.
+
+Docker was the tool that popularised application containers and remains the most widely used runtime in self-hosted and production environments.
+
+---
+
+## 2. Architecture
+
+Docker uses a client–server model:
+
+- **Docker client** (`docker`) – the CLI used to issue commands.
+- **Docker daemon** (`dockerd`) – the background service that manages images, containers, networks, and volumes.
+- **Registry** – where images are stored and fetched (default: Docker Hub).
+
+When you run `docker run nginx`, the client sends a request to the daemon, which pulls the image from the registry if not already present, then starts the container.
+
+---
+
+## 3. Images
+
+### 3.1 Dockerfile
+
+A `Dockerfile` is a plain text file that describes how to build an image step by step.
+
+```dockerfile
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*
+
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]
+```
+
+Each instruction creates a new layer. Docker caches layers to speed up rebuilds – only layers that change (and everything after them) are re-executed.
+
+### 3.2 Building Images
+
+Images are built with `docker build`. Common flags:
+
+- `-t name:tag` – assign a name and tag.
+- `-f Dockerfile.custom` – specify a non-default Dockerfile.
+- `--no-cache` – ignore cached layers and rebuild from scratch.
+
+See [Docker / Commands](./Commands.md) for practical examples.
+
+### 3.3 Image Tags
+
+A tag identifies a specific version of an image: `nginx:1.25`, `nginx:stable`, or `nginx:latest`. Using explicit version tags is safer than `latest`, which always points to the most recent build and can change without notice.
+
+---
+
+## 4. Containers
+
+### 4.1 Lifecycle
+
+A container moves through several states:
+
+- **Created** – exists but has not started yet.
+- **Running** – the main process is active.
+- **Paused** – processes are suspended.
+- **Stopped / Exited** – the main process has ended.
+- **Removed** – the container is deleted; its writable layer is discarded.
+
+Only data stored in volumes or bind mounts survives beyond the container's removal.
+
+### 4.2 Common Run Flags
+
+| Flag                       | Purpose                                      |
+|----------------------------|----------------------------------------------|
+| `-d`                       | Detached mode (run in background)            |
+| `--name`                   | Assign a name to the container               |
+| `-p host:container`        | Publish a port to the host                   |
+| `-v host:container`        | Bind mount a directory or attach a volume    |
+| `-e KEY=VALUE`             | Set an environment variable                  |
+| `--restart unless-stopped` | Restart policy (survives reboots)            |
+| `--network`                | Attach to a specific Docker network          |
+
+---
+
+## 5. Volumes and Bind Mounts
+
+### 5.1 Named Volumes
+
+Volumes are managed by Docker and stored under `/var/lib/docker/volumes/`. They persist across container restarts and removals (unless explicitly deleted with `docker volume rm`).
+
+Use named volumes for data that must survive the container lifecycle: databases, application state, generated files.
+
+### 5.2 Bind Mounts
+
+Bind mounts surface a specific host path inside the container at a chosen mount point. They are suitable for:
+
+- Configuration files managed on the host.
+- Application data stored in a known host directory.
+
+Unlike named volumes, bind mounts depend directly on the host filesystem layout and are not tracked by Docker.
+
+---
+
+## 6. Networks
+
+Docker provides several network drivers:
+
+- **bridge** (default) – containers on the same bridge network can reach each other by container name. User-defined bridges (created explicitly) support automatic DNS resolution between containers.
+- **host** – the container shares the host's network stack directly; no network isolation is applied.
+- **none** – the container has no network access.
+- **overlay** – multi-host communication (Swarm/Kubernetes); not relevant for single-host setups.
+
+Creating a dedicated user-defined bridge network for each application stack is the recommended approach; it avoids exposing containers to unrelated services.
+
+---
+
+## 7. Docker Compose
+
+Docker Compose defines a multi-container application in a single `compose.yaml` (or `docker-compose.yml`) file.
+
+A minimal example:
+
+```yaml
+services:
+  app:
+    image: myapp:latest
+    restart: unless-stopped
+    ports:
+      - "8080:80"
+    volumes:
+      - /host/appdata/myapp:/data
+    environment:
+      - APP_ENV=production
+
+  db:
+    image: postgres:16
+    restart: unless-stopped
+    volumes:
+      - db_data:/var/lib/postgresql/data
+    environment:
+      - POSTGRES_PASSWORD=secret
+
+volumes:
+  db_data:
+```
+
+Key Compose concepts:
+
+- **services** – each service becomes one container.
+- **volumes** – named volumes declared here are managed by Docker.
+- **networks** – Compose creates a default user-defined bridge network per project; services reach each other by service name.
+- **env_file** – loads environment variables from a file; useful to keep secrets out of `compose.yaml`.
+
+For Compose command examples, see [Docker / Commands](./Commands.md).
+
+---
+
+## 8. Relation to Other Documents
+
+- [Containerisation / Basics](../Basics.md)
+- [Docker / Commands](./Commands.md)
+- [Home-Server / Docker Stacks](../../Home-Server/Containers/Stacks.md)