Dockerfile: Update to the bionic-20200713 tag

Move up to the latest tag to get the usual related security fixes that
brings in.

Signed-off-by: Tom Rini <trini@konsulko.com>

Dockerfile

@@ -2,7 +2,7 @@
 # This Dockerfile is used to build an image containing basic stuff to be used
 # to build U-Boot and run our test suites.
 
-FROM ubuntu:bionic-20200630
+FROM ubuntu:bionic-20200713
 MAINTAINER Tom Rini <trini@konsulko.com>
 LABEL Description=" This image is for building U-Boot inside a container"