Update Dockerfile to fix H7RF builds.
The H7RF target (and possibly others?) needs the xxd utility. This is not included in the current Docker file. I was not able to include xxd directly for some reason, but it is included as part of vim-common.

@AlessandroAU did you try:
FROM ubuntu:16.04
COPY xxd /usr/bin/xxd
Also maybe we should update Ubuntu to 20.04.
I think I tried that and it wasn't able to find the package.
btw, this isn't specific to the H7RF, it's specific to all the EXST targets, such as the SPRacingH7EXTREME/NANO/ZERO
This issue / pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within a week.