Support overriding the whole postfix config

[bendem]

This is related to #45, but I'd like to override the whole file. We have specific templates depending on which mail server we use and exposing the config keys one by one is not going to cut it. Since the container modifies the file on start, we can't simply mount the file directly on the container.

[asimzeeshan]

@bendem modify the Dockerfile and put your custom configs in it