Add docker build script for rootfs_size.py
Add docker build script for rootfs_size.py

Change-Id: Ib287ffd33a5333b6b0a1fdf64aa1a392aff48392
Signed-off-by: Michael Shepos <f264ce3af22ce77e51cc4f4a2a4f43efd8cf4fed@us.ibm.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts