Add custom entrypoint to generate the configuration

[dabelenda]

Having to rebuild an image only to include the configuration is not ideal. This PR adds an rudimentary way to create the Caddyfile.

[abiosoft]

Firstly, thanks for this and sorry this is coming a bit late. I would like to mention that there is strong preference for non-breaking change and simplicity. However, I will try to have a look and see if this provides a better experience for users.

Thanks.

[dabelenda]

I made a change to the entrypoint, to make the PR a non-breaking change.