[FEATURE]: Ability to add custom config file out of the box

[runlevel5]

Describe the feature you'd like

My use-case requires a bit of modification to the config file and the current offerings of configurations via POSIX environment variables is lacking.

For example: To add /etc/datadog-agent/conf.d/postgres.d/conf.yaml file for AWS RDS monitoring, I have to make a custom Docker image.

Is your feature request related to a problem?

No

Describe alternatives you've considered

IMHO it would way better if the stock Docker image offers ways to mount Docker volume for config YAML files.

Additional context

No response