Use polling inside docker container

Without polling, file changes from a mounted volume may not be detected.
See similar [nodemon caveat](https://github.com/remy/nodemon#application-isnt-restarting).
Although `/.dockerenv` content [is not being used anymore](moby/libnetwork#815),
it still seems to be a reilable way to detect whether we are running inside
a Docker container.

source/elm-live.js

@@ -214,6 +214,7 @@ ${dim("elm-live:")}
   const watcher = chokidar.watch("**/*.elm", {
     ignoreInitial: true,
     followSymlinks: false,
+    usePolling: fs.existsSync("/.dockerenv"),
     ignored: "elm-stuff/generated-code/*"
   });