Remove our suggestions for not using the container cache, which stemmed
from the fact that our Dangerzone image was not reproducible. Now that
we have switched to Debian Stable and the Dockerfile is all we need to
reproducibly build the exact same container image, we can just use the
cache to speed up builds.
Update our release instructions with a way to run manual tasks via
`doit`. Also, add developer documentation on how to use `doit`, and some
tips and tricks.
