Dockerfile: add mkbootimg tool

mkbootimg tool is part of the Android project and it is
used to pack Android boot images such as boot image
and vendor_boot image.

Use the following command to run mkbootimg:
$ python3 -m mkbootimg

Add mkbootimg to the docker file

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 801bebf..9804b55 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -284,3 +284,7 @@
 RUN /bin/echo -e "\n[toolchain-alias]\nsh = sh2" >> ~/.buildman
 RUN /bin/echo -e "\nsandbox = x86_64" >> ~/.buildman
 RUN /bin/echo -e "\nx86 = i386" >> ~/.buildman;
+
+# Add mkbootimg tool
+RUN git clone https://android.googlesource.com/platform/system/tools/mkbootimg /home/uboot/mkbootimg
+ENV PYTHONPATH "${PYTHONPATH}:/home/uboot/mkbootimg"