Add newline when generating Dockerfile

Signed-off-by: Alessandro Di Federico <ale@rev.ng>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1610080146-14968-36-git-send-email-tsimpson@quicinc.com>
Message-Id: <20210114165730.31607-5-alex.bennee@linaro.org>