Add Docker + File providers user guide (#239)

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/user-guides/docker-file-providers.md

@@ -0,0 +1,57 @@
+# Docker + File providers
+
+It is possible to use several providers at the same time with Diun. This can be particularly useful if you want to
+analyze some images that you don't manage through a container.
+
+In this section we quickly go over a basic docker-compose file to run Diun using the [docker](../providers/docker.md)
+and [file](../providers/file.md) providers.
+
+## Setup
+
+Create a `docker-compose.yml` file that uses the official Diun image:
+
+```yaml
+version: "3.5"
+
+services:
+  diun:
+    image: crazymax/diun:latest
+    volumes:
+      - "./data:/data"
+      - "./custom-images.yml:/custom-images.yml:ro"
+      - "/var/run/docker.sock:/var/run/docker.sock"
+    environment:
+      - "TZ=Europe/Paris"
+      - "LOG_LEVEL=info"
+      - "LOG_JSON=false"
+      - "DIUN_WATCH_WORKERS=20"
+      - "DIUN_WATCH_SCHEDULE=0 */6 * * *"
+      - "DIUN_PROVIDERS_DOCKER=true"
+      - "DIUN_PROVIDERS_DOCKER_WATCHBYDEFAULT=true"
+      - "DIUN_PROVIDERS_FILE_FILENAME=/custom-images.yml"
+    restart: always
+```
+
+```yaml
+# /custom-images.yml
+- name: ghcr.io/crazy-max/diun
+  watch_repo: true
+- name: alpine
+  watch_repo: true
+- name: debian:stretch-slim
+- name: nginx:stable-alpine
+- name: traefik
+  watch_repo: true
+  include_tags:
+    - ^(0|[1-9]\d*)\..*-alpine
+```
+
+Here we use a minimal configuration to analyze **all running containers** (watch by default enabled) of
+your **local Docker** instance with the [Docker provider](../providers/docker.md) and also **custom images**
+through the [File provider](../providers/file.md) **every 6 hours**.
+
+That's it. Now you can launch Diun with the following command:
+
+```shell
+$ docker-compose up -d
+```

mkdocs.yml

@@ -106,6 +106,8 @@ nav:
     - File: providers/file.md
     - Kubernetes: providers/kubernetes.md
     - Swarm: providers/swarm.md
+  - User guides:
+    - Docker + File providers: user-guides/docker-file-providers.md
   - FAQ: faq.md
   - Changelog: changelog.md
   - Migration: