update compose & override to match new dockerfile

docker-compose.override.tmpl.yml

@@ -1,8 +1,9 @@
 version: '3.7'
 services:
   nginx:
-    environment:
-      - URL=a_url_unused
+    build:
+      args:
+        nginx_template_data: nginx.yaml
   tty-proxy:
     environment:
       - URL=tty.example.com

docker-compose.yml

@@ -1,7 +1,9 @@
 version: '3.7'
 services:
   nginx:
-    build: ./nginx/
+    build: 
+      context: ./nginx/
+    image: webserver
     container_name: webserver
     volumes:
       # - type: bind