No need to use two different base images. Debian buster is pulled later
anyway so we can just use it for the vault stage as well.
My reason for this change is partly to avoid redundancy and partly to
make it easier to build everything yourself. When all the build
environment is based on Debian than you just have to figure out how to
build a Debian Docker base image (ref:
https://github.com/ypid/docker-makefile).
Robin Schneider