Thanks to everyone for reporting this issue.
I have pending fix that is undergoing review and I'm hoping we can have a fix out in a few hours.
If you don't want to wait for the fix, you can manually change line 5 in service.dockerfile to this:
RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ $(grep -oP 'VERSION_CODENAME=\K\w+' /etc/os-release)-pgdg main" | tee /etc/apt/sources.list.d/pgdg.list; \``
The change is to replace stretch with $(grep -oP 'VERSION_CODENAME=\K\w+' /etc/os-release). Below is a diff to make the change easier to see.
I'll update this topic when the pending fix has been released.