Add TMDB env vars and rename Docker service

Add TMDB_API_KEY and TMDB_API_TOKEN to .env.example to document required TMDB credentials. Rename the Docker Compose service from `server` to `what-to-watch` in compose.yaml to align the service name with the project; also remove an extra trailing blank line.

compose.yaml

@@ -8,7 +8,7 @@
 # database or a cache. For examples, see the Awesome Compose repository:
 # https://github.com/docker/awesome-compose
 services:
-  server:
+  what-to-watch:
     build:
       context: .
     environment:
@@ -48,4 +48,3 @@ services:
 # secrets:
 #   db-password:
 #     file: db/password.txt
-