feat(image): make Nginx TCP port configurable

[peknur]

This adds possibility to listen different TCP ports using NGINX_PORT environment variable. This is useful if you want to run multiple instances of this container inside one pod, e.g.:

spec:
  containers:
    - image: ghcr.io/upcloudltd/hello
      name: hello80
      env:
        - name: NGINX_PORT
          value: "80"
    - image: ghcr.io/upcloudltd/hello
      name: hello8080
      env:
        - name: NGINX_PORT
          value: "8080"
    - image: ghcr.io/upcloudltd/hello
      name: hello81
      env:
        - name: NGINX_PORT
          value: "81"

NGINX_PORT defaults to 80