Add 3.7.12-stretch tag

3.7 has been the latest supported version on stretch, but as of 3.7.12 there doesn't seem to be a 3.7.12-stretch image built. It would be great if 3.7 images could continue to be built for stretch for the remaining lifetime of 3.7.

With Debian Bullseye's release in August the Stretch variants were removed as we only maintain two tracks per distro, given the large load of compilation that happens for each variation #633