OK. Trying to piece together what I did... Credit to user 'nailer' who helped figure it out.
So... run the following inside the shell in the docker image:
echo "Europe/London" > /config/etc/timezone
if it complains and fails, you might need to do this first before repeating
mkdir /config/etc
Then create a new Volume, file mapping from the file you just created, which will be in somewhere like /Users/MyUserName/Documents/dslstats/config/etc/timezone and map it to /etc/timezone within the Docker container. Im not sure if you an do this in Kitematic. If not, you will need to add it to the original long command line I gave you as another mapping argument to go along with the existing one for the 'config' folder.
Finally, back in the Docker shell,
dpkg-reconfigure -f noninteractive tzdata
And restart the container.
Good luck!