Don't write out the config to /

[gracenoah]

Currently the config is written to a file in /. This requires

1. That the container runs as root
2. That the container's filesystem is writable

See https://github.com/satoshipay/docker-stellar-core/blob/master/confd/conf.d/stellar-core.cfg.toml#L3

This should be changed to any other subdirectory, such as /config so that the user of this image can make the container filesystem read only and/or control the user that it runs under.