How to use nginx as a sidecar container for IIS in Kubernetes?

I have a strange result from using nginx and IIS server together in single Kubernetes pod. It seems to be an issue with nginx.conf. If I bypass nginx and go directly to IIS, I see the standard landing page -

However when I try to go through the reverse proxy I see this partial result -

Here are the files:

nginx.conf:

events {
  worker_connections  4096;  ## Default: 1024
}

http{
    server {
       listen 81;
	   #Using variable to prevent nginx from checking hostname at startup, which leads to a container failure / restart loop, due to nginx starting faster than IIS server. 
	   set $target "http://127.0.0.1:80/"; 
	   location / { 
	      proxy_pass $target; 
	   }
     }
}

deployment.yaml:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    ...
  name: ...
spec:
  replicas: 1
  selector:
    matchLabels:
      pod: ...
  template:
    metadata:
      labels:
        pod: ...
      name: ...
    spec:
      containers:
        - image: claudiubelu/nginx:1.15-1-windows-amd64-1809
          name: nginx-reverse-proxy
          volumeMounts:
            - mountPath: "C:/usr/share/nginx/conf" 
              name: nginx-conf
          imagePullPolicy: Always
        - image: some-repo/proprietary-server-including-iis
          name: ...
          imagePullPolicy: Always
      nodeSelector:
        kubernetes.io/os: windows
      imagePullSecrets:
        - name: secret1
      volumes:
        - name: nginx-conf
          persistentVolumeClaim:
            claimName: pvc-nginx

Mapping the nginx.conf file from a volume is just a convenient way to rapidly test different configs. New configs can be swapped in using kubectl cp ./nginx/conf nginx-busybox-pod:/mnt/nginx/.

Busybox pod (used to access the PVC):

apiVersion: v1
kind: Pod
metadata:
  name: nginx-busybox-pod
  namespace: default
spec:
  containers:
    - image: busybox
      command:
        - sleep
        - "360000"
      imagePullPolicy: Always
      name: busybox
      volumeMounts:
       - name: nginx-conf
         mountPath: "/mnt/nginx/conf"
  restartPolicy: Always
  volumes:
    - name: nginx-conf
      persistentVolumeClaim:
        claimName: pvc-nginx
  nodeSelector:
    kubernetes.io/os: linux

And lastly the PVC:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-nginx
spec:
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 100Mi
  storageClassName: azurefile  

Any ideas why?

After some testing, here is a working nginx.conf -

http{  
    server {  
       listen 81;  
       set $target "http://127.0.0.1:80"; 
       location / { 
          proxy_pass $target; 
          proxy_set_header Host $host;
       }
     }
}

• New directive - proxy_set_header Host $host;
• Trailing slash removed from the target variable used by the proxy_pass directive.
• (Specific to my application) Other endpoints on the server are better reachable using $host:$server_port in place of $host. This is caused by the app server redirecting incoming requests to different URIs, losing the proxy's port (81) in the process.