Allow cassandra to be configured via config map on K8s

[abinet]

We use cassandra images on K8s and need some additional configuration changes in cassandra.yaml file.
It is standard approach on K8s to use config map for this purpose which has all the configuration settings and being mounted into the pod.

But in case when docker-entrypoint.sh changes cassandra.yaml during startup replacing some values with the environment variables mounting cannot be used.

The idea is to use the mounted file as a template and let docker-entrypoint.sh to change the copy of the original configuration stored in CASSANDRA_CONFIG_TEMPLATE_FOLDER/cassandra.yaml. When CASSANDRA_CONFIG_TEMPLATE_FOLDER is not set, script uses the old behavior and modifies configuration directly.

So I can share original configuration to multiple containers:

docker run -d -e CASSANDRA_NUM_TOKENS=100 -e CASSANDRA_CLUSTER_NAME=mycluster -e CASSANDRA_CONFIG_TEMPLATE_FOLDER=/templates -v ${PWD}/cassandra.yaml:/templates/cassandra.yaml:ro cassandra

docker run -d -e CASSANDRA_NUM_TOKENS=100 -e CASSANDRA_CLUSTER_NAME=mycluster -e CASSANDRA_CONFIG_TEMPLATE_FOLDER=/templates -v ${PWD}/cassandra.yaml:/templates/cassandra.yaml:ro cassandra

...
docker run -d -e CASSANDRA_NUM_TOKENS=100 -e CASSANDRA_CLUSTER_NAME=mycluster -e CASSANDRA_CONFIG_TEMPLATE_FOLDER=/templates -v ${PWD}/cassandra.yaml:/templates/cassandra.yaml:ro cassandra

[abinet]

This PR addresses the issues like:

#58
#110

[yosifkit]

Why is this required over doing it on the command line with a shell script?

$ docker run -d -e CASSANDRA_NUM_TOKENS=100 -e CASSANDRA_CLUSTER_NAME=mycluster -v ${PWD}/cassandra.yaml:/templates/cassandra.yaml:ro cassandra sh -c 'cp /templates/cassandra.yaml $CASSANDRA_CONFIG/cassandra.yaml && exec /docker-entrypoint.sh cassandra -f'

[abinet]

Sure, you can do this via command line
My solution just provides a shortcut for a very common use case.

[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.