Read Console credentials from environment variables first

[toqueteos]

New contributor, I couldn't find any issues talking about this so I went for it directly.

I'll add whatever is missing/required.

The fact that the default credentials are known and always set is not very good from a security standpoint.

Having to mess around with config files and the entrypoint of the container seems a bit too tedious for something that could be resolved via, at most, two environment variables.

Other solutions:

• Some people might not like getEnvOr existing in server/config.go, if at all, we could not set a fallback value by just using os.Getenv instead and modify all the Dockerfiles in build/ folder to set the default value(s) there, although that adds chore tasks for new architectures or updates happening in said files.
• Modify nakama/flags to support reading from env vars if some struct tag is set (something like env:"KEY_NAME").

[CLAassistant]

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