environment variables with placeholders not working

[im-pratham]

Steps to reproduce -

1. Lets say you have property like CONTROL_CENTER_STREAMS_SSL_KEYSTORE_PASSWORD with value ${KEYSTORE_PASSWORD}
2. the dub template command will create admin.properties file with ssl.keystore.passsword=${KEYSTORE_PASSWORD}
3. The https://github.com/confluentinc/common-docker/blob/master/utility-belt/src/main/java/io/confluent/admin/utils/cli/KafkaReadyCommand.java class only takes properties as key value pair and does no substitution. Hence the docker image fails at step "Checking Kafka Ready".

Background -

I encountered this scenario when I am creating ssl certs using automatic cert provisioning as keystore password is injected as env variable by another system. So I need to just configure the properties using placeholders.

Possible Solutions -

We can add the logic in any of the 3 places mentioned above, 1. dub command (can be good choice), 2. admin.properties.template or in 3. KafkaReadyCommand.java.

Let me know your thoughts.