docker: Make it easier to run on-off commands

Set ENTRYPOINT to the Pat executable, and use CMD to provide default
arguments. By doing this, users can run on-off commands with docker as
if the app was running natively on the host.

E.g. `docker compose run pat interactive`.

Dockerfile

@@ -18,4 +18,5 @@ ENV XDG_CONFIG_HOME=/app
 ENV XDG_DATA_HOME=/app
 ENV XDG_STATE_HOME=/app
 EXPOSE 8080
-ENTRYPOINT ["/bin/pat", "http", "--addr", ":8080"]
+ENTRYPOINT ["/bin/pat"]
+CMD ["http", "--addr", ":8080"]