Add Xenial, Bionic, CentOS 7 and 8 workflows with OpenSSL-Dynamic builds
nightly.

Omitting an entrypoint.sh to configure the container and instead
depending on docker primitives allows us to be more portable.  (If a
distribution uses a different mechanism for adding users, we need not
have multiple entrypoint.sh files or invariants within it; instead we
can configure that in the dockerfile itself along with all the other
distribution specific components.)

Ninja is not installed by default on the macOS machines; stop trying to
use it.  Instead use `make -j` which should be roughly equivalent in
performance but supported everywhere.