You want Ingress. There are several options available (Istio, nginx, traefik, etc). I like using nginx and it's really easy to install and work with. Installation steps can be found at kubernetes.github.io.

Once the Ingress Controller is installed, you want to make sure you've exposed it with a Service with type=LoadBalancer. Next, if you are using Google Cloud DNS, set up a wildcard entry for your domain with an A record pointing to the external IP address of your Ingress Controller's Service. In your case, it would be *.app.mydomain.com.

So now all of your traffic to app.mydomain.com is going to that load balancer and being handled by your Ingress Controller, so now you need to add Service and Ingress Entities for any service you want.

apiVersion: v1
kind: Service
metadata:
  name: my-service1
spec:
  selector:
    app: my-app-1
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: ClusterIP

apiVersion: v1
kind: Service
metadata:
  name: my-service2
spec:
  selector:
    app: my-app2
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: ClusterIP

apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: name-virtual-host-ingress
spec:
  rules:
  - host: sub1.app.mydomain.com
    http:
      paths:
      - backend:
          serviceName: my-service1
          servicePort: 80
  - host: sub2.app.mydomain.com
    http:
      paths:
      - backend:
          serviceName: my-service2
          servicePort: 80

Routing shown is host based, but you just as easily could have handled those services as path based, so all traffic to app.mydomain.com/service1 would go to one of your deployments.

It could be a solution, for my case I need something more dynamic. I would not update the ingress each time I add a subdomain.

I've almost solved using an nginx proxy like this:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: my-subdomain-1
spec:
replicas: 1
strategy: {}
template:
    metadata:
    creationTimestamp: null
    labels:
        name: my-subdomain-1
        type: app
    spec:
    hostname: sub1
    subdomain: my-internal-host
    containers:
        - image: nginx:alpine
        name: nginx
        ports:
            - name: nginx
            containerPort: 80
            hostPort: 80
    restartPolicy: Always
status: {}
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: my-subdomain-2
spec:
replicas: 1
strategy: {}
template:
    metadata:
    creationTimestamp: null
    labels:
        name: my-subdomain-2
        type: app
    spec:
    hostname: sub2
    subdomain: my-internal-host
    containers:
        - image: nginx:alpine
        name: nginx
        ports:
            - name: nginx
            containerPort: 80
            hostPort: 80
    restartPolicy: Always
status: {}
---
apiVersion: v1
kind: ConfigMap
metadata:
name: nginx-config-dns-file
data:
nginx.conf: |
    server {
    listen       80;
    server\_name ~^(?.\*?)\\.;

    location / {
        proxy\_pass         http://$subdomain.my-internal-host;
        root   /usr/share/nginx/html;
        index  index.html index.htm;
    }

    error\_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
    }
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: my-proxy
spec:
replicas: 1
strategy: {}
template:
    metadata:
    creationTimestamp: null
    labels:
        name: my-proxy
        type: app
    spec:
    subdomain: my-internal-host
    containers:
        - image: nginx:alpine
        name: nginx
        volumeMounts:
            - name: nginx-config-dns-file
            mountPath: /etc/nginx/conf.d/default.conf.test
            subPath: nginx.conf
        ports:
            - name: nginx
            containerPort: 80
            hostPort: 80
    volumes:
        - name: nginx-config-dns-file
        configMap:
            name: nginx-config-dns-file
    restartPolicy: Always
status: {}
---
apiVersion: v1
kind: Service
metadata:
name: my-internal-host
spec:
selector:
    type: app
clusterIP: None
ports:
    - name: sk-port
    port: 80
    targetPort: 80
---
apiVersion: v1
kind: Service
metadata:
name: sk-expose-dns
spec:
ports:
    - port: 80
selector:
    name: my-proxy
type: LoadBalancer

I did understand that I need the service 'my-internal-host' to allow all the deployments to see each other internally. The problem now is only the proxy_pass of nginx, if I change it with 'proxy_pass http://sub1.my-internal-host;' it works, but not with the regexp var.

The problem is related to the nginx resolver.