Add check endpoint which can be used with nginx' auth_request function (#266)

* Add check endpoint which can be used with nginx' auth_request function

* feat(cmd): allow configuring redirect domains

* test: add test environment for the nginx_auth PR

This is a full local setup of the nginx_auth PR including HTTPS so that
it's easier to validate in isolation.

This requires an install of k3s (https://k3s.io) with traefik set to
listen on localhost. This will be amended in the future but for now this
works enough to ship it.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(cmd|lib): allow empty redirect domains variable

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(test): add space to target variable in anubis container

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs(admin): rewrite subrequest auth docs, make generic

* docs(install): document REDIRECT_DOMAINS flag

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(lib): clamp redirects to the same HTTP host

Only if REDIRECT_DOMAINS is not set.

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Co-authored-by: Xe Iaso <me@xeiaso.net>

test/nginx-external-auth/deployment.yaml

@@ -0,0 +1,50 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx-external-auth
+spec:
+  selector:
+    matchLabels:
+      app: nginx-external-auth
+  template:
+    metadata:
+      labels:
+        app: nginx-external-auth
+    spec:
+      volumes:
+      - name: config
+        configMap:
+          name: nginx-cfg
+      containers:
+      - name: www
+        image: nginx:alpine
+        resources:
+          limits:
+            memory: "128Mi"
+            cpu: "500m"
+          requests:
+            memory: "128Mi"
+            cpu: "500m"
+        ports:
+        - containerPort: 80
+        volumeMounts:
+        - name: config
+          mountPath: /etc/nginx/conf.d
+          readOnly: true
+      - name: anubis
+        image: ttl.sh/techaro/anubis-external-auth:latest
+        imagePullPolicy: Always
+        resources:
+          limits:
+            cpu: 500m
+            memory: 128Mi
+          requests:
+            cpu: 250m
+            memory: 128Mi
+        env:
+        - name: TARGET
+          value: " "
+        - name: REDIRECT_DOMAINS
+          value: nginx.local.cetacean.club
+
+