Allow proxysql entrypoint to work with helm config

[dcaputo-harmoni]

The current entrypoint script attempts to modify /etc/proxysql/proxysql.cnf using sed (replacing sensitive data such as usernames / passwords with environment variables) however when deploying proxysql as part of the pxc-db helm chart, where proxysql.configuration is specified, a configmap (mysql-db-proxysql) is created and mounted to the /etc/proxysql path, which is by definition read only.

As a result, the entrypoint script generates errors on startup, but also prevents the usage of the generic values such as admin:admin which would otherwise be replaced by environment variables in entrypoint.sh, forcing the hardcoding of these values in a helm config which is not secure.

The proposed change simply copies the config file form /etc/proxysql/proxysql.cnf to /etc/proxysql.cnf before performing the modifications, and then uses that config file when starting proxysql.

[it-percona-cla]

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