The default image: `gcr.io/prysmaticlabs/prysm/beacon-chain:latest`
already has the `http-` flags as per
https://github.com/prysmaticlabs/prysm/pull/14089

I also swapped the ports. Instead of using 3500 for the HTTP server and
4000 for the RPC server ( which is actually prysm's default behaviour).
The problem is that all the other CL clients are running their HTTP
beacon API under port 4000 ( `BEACON_HTTP_PORT_NUM= 4000`) . So I think
it makes sense to do the same for prysm here.