[docker] Don't overwrite /kafka-connect-ui/env.js if volume mounted

[OneCricketeer]

In Compose, I did this

    volumes:
      - ./kafka-connect-env.js:/kafka-connect-ui/env.js

But the file was unexpectedly overwritten.

Putting the file in read-only mode lets the container start, but it seems it's not able to connect to any of the clusters. Just giving a single CONNECT_URL via an environment variable with one of the URLs from the JS file works fine.