Update documentation to add parameter dockerfile

[julbrs]

This parameter is already available but not documented.

I use this parameter like this :

          <configuration>
            <dockerfile>Dockerfile.tomcat</dockerfile>
            <repository>${docker.repository}</repository>
            <tag>${project.version}</tag>
          </configuration>

It work well to create multiple image from a single project ! (for example with multiple maven profiles)

Thanks a lot for the hard work on this plugin,

[ktor]

I use it to build multiple images that share the same resources repository. Why is that simple pull request not merged yet ? :)