Switch build containers to Debian Stretch

[ddohler]

Build failures in #337 were apparently caused by a Chromium update from 71 to 72 in Debian Stretch. Switching to Jessie solved the problem because it maxes out at Chromium 57. However, LTS support for Jessie expires in June, 2020, so it would be good to get back on Stretch when possible. Hopefully, whatever this problem was will be fixed in subsequent Chromium releases. If not, we will need to look into alternative ways of solving the problem. One approach would be to push a built image to a repository and use that for testing.