Fix start command

Otherwise:
```
Error: Directory import '/srv/build' is not supported resolving ES modules imported from /srv/node_modules/@remix-run/serve/dist/cli.js
Did you mean to import /srv/build/index.js?
    at new NodeError (node:internal/errors:406:5)
    at finalizeResolution (node:internal/modules/esm/resolve:227:11)
    at moduleResolve (node:internal/modules/esm/resolve:845:10)
    at defaultResolve (node:internal/modules/esm/resolve:1043:11)
    at ModuleLoader.defaultResolve (node:internal/modules/esm/loader:383:12)
    at ModuleLoader.resolve (node:internal/modules/esm/loader:352:25)
    at ModuleLoader.getModuleJob (node:internal/modules/esm/loader:228:38)
    at ModuleLoader.import (node:internal/modules/esm/loader:315:34)
    at importModuleDynamically (node:internal/modules/cjs/loader:1164:37)
    at importModuleDynamicallyWrapper (node:internal/vm/module:431:21)
```

src/Dockerfile

@@ -39,4 +39,4 @@ COPY --from=build /srv/build /srv/build
 COPY --from=build /srv/public /srv/public
 
 # https://engineeringblog.yelp.com/2016/01/dumb-init-an-init-for-docker.html
-CMD [ "dumb-init", "./node_modules/.bin/remix-serve", "build" ]
+CMD [ "dumb-init", "./node_modules/.bin/remix-serve", "build/index.js" ]