Add automated image update examples to docs (#2369)

Add automated image update examples to docs

docs/references/automated-image-update.md

@@ -0,0 +1,87 @@
+# Automated deployment of new container images
+
+Flux can be used to automate container image updates in your cluster.
+Flux periodically scans the pods running in your cluster and builds a list of all container images.
+Using the image pull secrets, it connects to the container registries, pulls the images metadata
+and stores the image tag list in memcached.
+
+You can enable the automate image tag updates by annotating your deployments, statefulsets,
+daemonsets or cronjobs objects. You can also control what tags should be considered for an
+update by using glob, regex or semantic version expressions.
+
+> **Note:** that Flux only works with immutable image tags (`:latest` is not supported).
+Every image tag must be unique, for this you can use the Git commit SHA or semver when tagging images.
+
+## Examples
+
+What follows is a list of examples on how you can control the image update automation. If you're using Helm releases 
+please see the [Helm operator integration docs](helm-operator-integration.md).
+
+Turn on automation based on timestamp:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  annotations:
+    fluxcd.io/automated: "true"
+spec:
+  template:
+    spec:
+      containers:
+      - name: app
+        image: docker.io/org/my-app:1.0.0
+```
+
+The above configuration will make Flux update the `app` container when you push
+a new image tag, be it `my-app:1.0.1` or `my-app:9e3bdaf`.
+
+Restrict image updates with sem ver:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  annotations:
+    fluxcd.io/automated: "true"
+    fluxcd.io/tag.app: semver:~1.0
+spec:
+  template:
+    spec:
+      containers:
+      - name: app
+        image: docker.io/org/my-app:1.0.0
+```
+
+The above configuration will make Flux update the image when you push
+an image tag that matches the [semantic version](https://semver.org/)
+expression e.g `my-app:1.0.1` but not `my-app:1.2.0`. 
+
+Restrict image updates with glob and regex expressions:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  annotations:
+    fluxcd.io/automated: "true"
+    fluxcd.io/tag.sidecar: regex:^stg.*
+    fluxcd.io/tag.app: glob:dev-*
+spec:
+  template:
+    spec:
+      containers:
+      - name: sidecar
+        image: docker.io/org/my-proxy:stg-4s7bsgv
+      - name: app
+        image: docker.io/org/my-app:dev-9e3bdaf
+```
+
+The above configuration will make Flux update the `sidecar` when you push
+a tag for the `my-proxy` image that begins with `stg`.
+For the `app` container, Flux will update it when you push a tag for the
+`my-app` image that begins with `dev-`.
+
+To target a specific container the annotation format is `fluxcd.io/tag.<CONTAINER>: <TYPE>:<EXPRESSION>`.
+
+You can turn off the automation with `fluxcd.io/automated: "false"` or with `fluxcd.io/locked: "true"`.

docs/references/index.rst

@@ -13,5 +13,6 @@ References
    fluxyaml-config-files
    garbagecollection
    git-gpg
+   automated-image-update
    helm-operator-integration
    monitoring