GHA/docs: Containerized Setup

Add a new section to the documentation about installing in containerized
environments. Also, add a GitHub Action that pushes this new section to
Docker Hub to display it as the container README.

.github/workflows/sync-for-container.yml

@@ -0,0 +1,21 @@
+name: Sync For-Container.md to Docker Hub
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout HEAD
+        uses: actions/checkout@v4
+
+      - name: Sync For-Container.md
+        uses: ms-jpq/sync-dockerhub-readme@df5121ab3314dc86fd36863eb111ab986fc6c612
+
+        with:
+          username: icingaadmin
+          password: '${{ secrets.DOCKER_HUB_PERSONAL_TOKEN }}'
+          repository: icinga/icingadb
+          readme: ./doc/02-Installation.md.d/For-Container.md

doc/02-Installation.md.d/For-Container.md

@@ -0,0 +1,85 @@
+# Installing Icinga DB in Containers
+
+This image integrates Icinga DB into your containerized environment.
+
+## Usage
+
+```bash
+docker network create icinga
+
+docker run --rm -d \
+	--network icinga \
+	--name redis-icingadb \
+	redis
+
+docker run -d \
+	--network icinga \
+	--name mariadb-icingadb \
+	-e MYSQL_RANDOM_ROOT_PASSWORD=1 \
+	-e MYSQL_DATABASE=icingadb \
+	-e MYSQL_USER=icingadb \
+	-e MYSQL_PASSWORD=123456 \
+	mariadb
+
+docker run -d \
+	--network icinga \
+	--restart always \
+	--name icingadb \
+	-e ICINGADB_REDIS_HOST=redis-icingadb \
+	-e ICINGADB_REDIS_PORT=6379 \
+	-e ICINGADB_REDIS_PASSWORD=123456 \
+	-e ICINGADB_DATABASE_HOST=mariadb-icingadb \
+	-e ICINGADB_DATABASE_PORT=3306 \
+	-e ICINGADB_DATABASE_DATABASE=icingadb \
+	-e ICINGADB_DATABASE_USER=icingadb \
+	-e ICINGADB_DATABASE_PASSWORD=123456 \
+	icinga/icingadb
+```
+
+The container doesn't need any volumes and takes the environment variables.
+
+### Environment Variables
+
+The environment variables consists of the key `ICINGADB_DATABASE_` and the configuration option name like `HOST` in uppercase.
+
+```
+ICINGADB_DATABASE_HOST=mariadb-icingadb
+```
+
+For more options, see the [Icinga DB configuration documentation].
+
+### Connect via TLS
+
+```bash
+docker run -d \
+	--network icinga \
+	--restart always \
+	--name icingadb \
+	-e ICINGADB_REDIS_HOST=redis-icingadb \
+	-e ICINGADB_REDIS_PORT=6379 \
+	-e ICINGADB_REDIS_PASSWORD=123456 \
+	-e ICINGADB_REDIS_TLS=true \
+	-e ICINGADB_REDIS_CERT='[PEM-encoded content or path to a file (in a volume)]' \
+	-e ICINGADB_REDIS_KEY='[PEM-encoded content or path to a file (in a volume)]' \
+	-e ICINGADB_REDIS_CA='[PEM-encoded content or path to a file (in a volume)]' \
+	-e ICINGADB_DATABASE_HOST=mariadb-icingadb \
+	-e ICINGADB_DATABASE_PORT=3306 \
+	-e ICINGADB_DATABASE_DATABASE=icingadb \
+	-e ICINGADB_DATABASE_USER=icingadb \
+	-e ICINGADB_DATABASE_PASSWORD=123456 \
+	-e ICINGADB_DATABASE_TLS=true \
+	-e ICINGADB_DATABASE_CERT='[PEM-encoded content or path to a file (in a volume)]' \
+	-e ICINGADB_DATABASE_KEY='[PEM-encoded content or path to a file (in a volume)]' \
+	-e ICINGADB_DATABASE_CA='[PEM-encoded content or path to a file (in a volume)]' \
+	icinga/icingadb
+```
+
+## Build it yourself
+
+```bash
+git clone https://github.com/Icinga/icingadb.git
+
+docker build -t icingadb --file Containerfile .
+```
+
+[Icinga DB configuration documentation]: https://icinga.com/docs/icingadb/latest/doc/03-Configuration/