Docker fully qualified image names (#15689)

* Fully qualified docker image names for the main Dockerfile and Complement related.

* Fully qualified docker image names for Dockerfiles associated with building Debian release artifacts.

This one is harder and is separate from the other commit in case it wasn't correct or was unwanted. I decided to
do the expansion on the docker images in the Dockerfile itself, instead of the various source places that build
which distribution that is selected, as it would have been more invasive with the scripts breaking up the string
for tagging and such. This one is untested.

* Changelog

* Update docker/Dockerfile-workers

* Update docker/complement/Dockerfile

---------

Co-authored-by: reivilibre <olivier@librepush.net>

docker/Dockerfile-dhvirtualenv

@@ -24,7 +24,7 @@ ARG distro=""
 # https://launchpad.net/~jyrki-pulliainen/+archive/ubuntu/dh-virtualenv, but
 # it's not obviously easier to use that than to build our own.)
 
-FROM ${distro} as builder
+FROM docker.io/library/${distro} as builder
 
 RUN apt-get update -qq -o Acquire::Languages=none
 RUN env DEBIAN_FRONTEND=noninteractive apt-get install \
@@ -55,7 +55,7 @@ RUN cd /dh-virtualenv && DEB_BUILD_OPTIONS=nodoc dpkg-buildpackage -us -uc -b
 ###
 ### Stage 1
 ###
-FROM ${distro}
+FROM docker.io/library/${distro}
 
 # Get the distro we want to pull from as a dynamic build variable
 # (We need to define it in each build stage)