tests/docker: use explicit docker.io registry

It is good practice to use an explicit registry for referencing the base
image. This is because some distros will inject their own registries
into the search path. For example registry.fedoraproject.org comes ahead
of docker.io. Using an explicit registry avoids wasting time querying
multiple registries for images that they won't have.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210623142245.307776-5-berrange@redhat.com>
Message-Id: <20210709143005.1554-13-alex.bennee@linaro.org>

tests/docker/dockerfiles/centos8.docker

@@ -1,4 +1,4 @@
-FROM centos:8.3.2011
+FROM docker.io/centos:8
 
 RUN dnf -y update
 ENV PACKAGES \

tests/docker/dockerfiles/debian-xtensa-cross.docker

@@ -5,7 +5,7 @@
 # using a prebuilt toolchains for Xtensa cores from:
 # https://github.com/foss-xtensa/toolchain/releases
 #
-FROM debian:stretch-slim
+FROM docker.io/library/debian:stretch-slim
 
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt install -yy eatmydata && \

tests/docker/dockerfiles/debian10.docker

@@ -7,7 +7,7 @@
 # On its own you can't build much but the docker-foo-cross targets
 # build on top of the base debian image.
 #
-FROM debian:buster-slim
+FROM docker.io/library/debian:buster-slim
 
 # Duplicate deb line as deb-src
 RUN cat /etc/apt/sources.list | sed "s/^deb\ /deb-src /" >> /etc/apt/sources.list

tests/docker/dockerfiles/debian11.docker

@@ -8,7 +8,7 @@
 # On its own you can't build much but the docker-foo-cross targets
 # build on top of the base debian image.
 #
-FROM debian:bullseye-slim
+FROM docker.io/library/debian:bullseye-slim
 
 # Duplicate deb line as deb-src
 RUN cat /etc/apt/sources.list | sed "s/^deb\ /deb-src /" >> /etc/apt/sources.list

tests/docker/dockerfiles/ubuntu.docker

@@ -9,7 +9,7 @@
 # system won't pick up that it has changed.
 #
 
-FROM ubuntu:20.04
+FROM docker.io/library/ubuntu:20.04
 ENV PACKAGES \
     ccache \
     clang \

tests/docker/dockerfiles/ubuntu1804.docker

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM docker.io/library/ubuntu:18.04
 ENV PACKAGES \
     ccache \
     clang \

tests/docker/dockerfiles/ubuntu2004.docker

@@ -1,4 +1,4 @@
-FROM ubuntu:20.04
+FROM docker.io/library/ubuntu:20.04
 ENV PACKAGES flex bison \
     bsdmainutils \
     ccache \