Add instructions with hints for testing AppImage build scripts

[andy5995]

I wrote some comments in one script but haven't made anything formal yet:

# To test this script locally, from the source root directory,
# you can use:
#
# docker run -it --rm -e HOSTUID=$(id -u) -e \
# -v $PWD:/workspace -w /workspace andy5995/linuxdeploy:v2-focal bash
#
# Normally 'bash' can be replaced with the /path/to/script but starting
# the container from a shell means you can run the script manually in the
# container and then fix any problems, then re-run the script (hint: the
# dependencies will be installed the first time, and not have to be installed
# on subsequent attempts).