private String extractDockerFilePath(DockerComposeServiceWrapper mapper, File parentDir) {
        if (mapper.requiresBuild()) {
            File buildDir = new File(mapper.getBuildDir());
            String dockerFile = mapper.getDockerfile();
            if (dockerFile == null) {
                dockerFile = "Dockerfile";
            }
            File ret = new File(buildDir, dockerFile);
            return ret.isAbsolute() ? ret.getAbsolutePath() : new File(parentDir, ret.getPath()).getAbsolutePath();
        } else {
            return null;
        }
    }
Docker Compose handler for allowing a docker-compose file to be used
to specify the docker images.
src/main/java/io/fabric8/maven/docker/config/handler/compose/DockerComposeConfigHandler.java