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-workers

@@ -7,7 +7,7 @@ ARG FROM=matrixdotorg/synapse:$SYNAPSE_VERSION
 # target image. For repeated rebuilds, this is much faster than apt installing
 # each time.
 
-FROM debian:bullseye-slim AS deps_base
+FROM docker.io/library/debian:bullseye-slim AS deps_base
     RUN \
        --mount=type=cache,target=/var/cache/apt,sharing=locked \
        --mount=type=cache,target=/var/lib/apt,sharing=locked \
@@ -21,7 +21,7 @@ FROM debian:bullseye-slim AS deps_base
 # which makes it much easier to copy (but we need to make sure we use an image
 # based on the same debian version as the synapse image, to make sure we get
 # the expected version of libc.
-FROM redis:6-bullseye AS redis_base
+FROM docker.io/library/redis:6-bullseye AS redis_base
 
 # now build the final image, based on the the regular Synapse docker image
 FROM $FROM