It turns out that some popular libaries still have problems with OpenSSL
1.1, particularly `postgres` 0.15. So we'll build two sets of images
now.
We also explain the new image tagging scheme, and fix a bug wher the
test script didn't test the newly built image.
We update our base image to Ubuntu 18.04, and update all of our
dependencies to the latest released versions. This includes:
- gcc-multilib-arm-linux-gnueabihf (latest distro version)
- mdbook 0.2.1 (0.2.3 is out, but there's no binary build)
- OpenSSL 1.1.1b
- libpq 11.2
