docs: add Kubernetes examples & documentation (#402)

Fix #396

k8s/README.md

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+# Kubernetes
+
+This directory has example plain Kubernetes manifests for running DDNS-updater in Kubernetes.
+
+The Manifests have additional [Kustomize](https://kustomize.io/) overlays, which can be used to add an [ingress-route](https://kubernetes.io/docs/concepts/services-networking/ingress/) to ddns-updater.
+
+1. Download the template files from the [`base` directory](base). For example with:
+
+    ```sh
+    curl -O https://raw.githubusercontent.com/qdm12/ddns-updater/master/k8s/base/deployment.yaml
+    curl -O https://raw.githubusercontent.com/qdm12/ddns-updater/master/k8s/base/secret-config.yaml
+    curl -O https://raw.githubusercontent.com/qdm12/ddns-updater/master/k8s/base/service.yaml
+    curl -O https://raw.githubusercontent.com/qdm12/ddns-updater/master/k8s/base/kustomization.yaml
+    ```
+
+1. Modify `secret-config.yaml` as described in the [project readme](../README#configuration)
+1. Use [kubectl](https://kubernetes.io/docs/reference/kubectl/) to apply the manifest:
+
+    ```sh
+    kubectl apply -k .
+    ```
+
+1. Connect the the web UI with a [kubectl port-forward](https://kubernetes.io/docs/tasks/access-application-cluster/port-forward-access-application-cluster/)
+
+    ```sh
+    kubectl port-forward svc/ddns-updater 8080:80
+    ```
+
+The web UI should now be available at [http://localhost:8080](http://localhost:8080).
+
+## Advanced usage
+
+Kustomize overlays can extend the installation:
+
+* [overlay/with-ingress](overlay/with-ingress/) - Basic **HTTP** Ingress ressource
+* [overlay/with-ingress-cert-manager](overlay/with-ingress-cert-manager/) - Basic **HTTPS** Ingress ressource which uses [cert-manager](https://github.com/cert-manager/cert-manager) to create certificates.
+
+To install with the overlay **just change dirctory in the overlay folder you want to install** and hit `kubectl apply -k .` .

k8s/base/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: ddns-updater
+  labels:
+    app: ddns-updater
+spec:
+  selector:
+    matchLabels:
+      app: ddns-updater
+  template:
+    metadata:
+      labels:
+        app: ddns-updater
+    spec:
+      containers:
+        - name: ddns
+          image: qmcgaw/ddns-updater:latest
+          envFrom:
+            - secretRef:
+                name: ddns-updater-config
+          ports:
+            - containerPort: 8000

k8s/base/kustomization.yaml

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+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - deployment.yaml
+  - secret-config.yaml
+  - service.yaml

k8s/base/secret-config.yaml

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+apiVersion: v1
+kind: Secret
+metadata:
+  name: ddns-updater-config
+type: Opaque
+stringData:
+  CONFIG: '{"settings":[{"provider":"ddnss","provider_ip":true,"domain":"YOUR-DOMAIN","host":"@","username":"YOUR-USERNAME","password":"YOUR-PASSWORD","ip_version":"ipv4"}]}'

k8s/base/service.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: ddns-updater
+spec:
+  selector:
+    app: ddns-updater
+  type: ClusterIP
+  ports:
+    - name: http
+      port: 80
+      targetPort: 8000

k8s/overlay/with-ingress-tls-cert-manager/ingress.yaml

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+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  annotations:
+    cert-manager.io/cluster-issuer: default
+    ingress.kubernetes.io/force-ssl-redirect: "true"
+  name: ddns-updater
+spec:
+  rules:
+    - host: localhost
+      http:
+        paths:
+          - path: /
+            pathType: ImplementationSpecific
+            backend:
+              service:
+                name: ddns-updater
+                port:
+                  number: 80
+  tls:
+    - hosts:
+        - localhost
+      secretName: ddns-updater-tls

k8s/overlay/with-ingress-tls-cert-manager/kustomization.yaml

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+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - ../../base
+  - ingress.yaml
+
+# this patches the hostname of the Ingress
+patches:
+  - patch: |-
+      - op: replace
+        path: /spec/rules/0/host
+        value: localhost # add your fqdn
+      - op: replace
+        path: /spec/tls/0/hosts/0
+        value: localhost # add your fqdn
+    target:
+      kind: Ingress
+      name: ddns-updater

k8s/overlay/with-ingress/ingress.yaml

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+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: ddns-updater
+spec:
+  rules:
+    - host: localhost
+      http:
+        paths:
+          - path: /
+            pathType: ImplementationSpecific
+            backend:
+              service:
+                name: ddns-updater
+                port:
+                  name: http

k8s/overlay/with-ingress/kustomization.yaml

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+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - ../../base
+  - ingress.yaml
+
+# this patches the hostname of the Ingress
+patches:
+  - patch: |-
+      - op: replace
+        path: /spec/rules/0/host
+        value: localhost # add your fqdn
+    target:
+      kind: Ingress
+      name: ddns-updater