Allow custom configuration options via env variables

[lewisdiamond]

This would make it easy to override any configuration.

For example:

CASSANDRA_CFG_auto_snapshot=false

The script that already seds the config could just check for a pattern in the environment variables and apply whatever config the user needs.

Makes it easy to work with Kubernetes, docker-compose, etc.

[gzeskas]

What is the status of this issue ?
It will allow for example override default Auth method, that now it's impossible to do.

[evanstucker-hates-2fa]

I need to be able to configure authentication. Can we place merge one of the various pull requests that fixes this?

[cchet]

I agree, why not supporting all configuration parameters via env file, or at least a possibility to point to a custom configuration file location via env var ?

[tianon]

I'm sorry, but honestly if we're going to do anything more with environment variables it would likely be in the opposite direction (which we won't do due to the backwards compatibility implications) -- modifying a YAML file via shell script is very fragile, and setting complex configuration in this way is not great. For custom configuration, the only/best recommendation is to provide an appropriate cassandra.yaml file to your running instance (via a new image with FROM / COPY in a short Dockerfile, via Docker Configs, via bind mount, etc).

Closing in favor of #160 + docker-library/docs#1317.