Simplify custom files copying

[augi]

To copy custom file to the image, something like this is required now:

dockerJava {
    image = imageName
    dockerBuildDirectory = projectDir
    dockerfileLines = [ "COPY deploy/kafka-client-ssl.truststore.jks /var/kafka-client-ssl.truststore.jks" ]
}

It would be nice to have a syntax to specify files to copy to Docker image, or just to the Docker working directory.