Hi, as mentioned in the other thread, the way Docker works, changes are not persisted over container restarts. You you have to add your changes to your Dockerfile / template.
Would all your devices have the same /etc/default/gpsd settings? Because then you can have copy of that gpsd file in your git repository, and copy it to the right place in the Dockerfile with
COPY gpsd /etc/default/gpsd
Or there are other ways too (modifying things within a RUN ... step, for example, or dynamically generating the file on container startup), but it depends on your exact use case.
Hope this helps some!