Add 3.7.12-stretch tag

[johnthagen]

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.

[wglambert]

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