Support for changing clamd settings without volume mounting full config

[sopelt]

Hi, thanks for providing an official docker image. Before that was available we looked at third party clamav docker images and were quite happy with the concept of having multiple environment variables that allowed changing setting (e.g. scan time/size limits etc.) just by changing a container environment variable. This allows for stateless deployment/operation (no volume mount for /etc/clam...). Has there ever been demand for/discussion about such a change? As far as I understand clamd uses clamd.conf as its exclusive source for options meaning the environment variables would have to be injected (e.g. via sed) in the entrypoint before starting clamd. If you would be open to such an inclusion I could provide a draft PR/work on that.