tests/docker: update and flatten debian-alpha-cross

Update to the latest stable Debian. While we are at it flatten into a
single dockerfile. We really don't need the rest of the stuff from
the QEMU base image just to compile test images.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220914155950.804707-9-alex.bennee@linaro.org>

.gitlab-ci.d/container-cross.yml

 alpha-debian-cross-container:
   extends: .container_job_template
   stage: containers
-  needs: ['amd64-debian10-container']
   variables:
     NAME: debian-alpha-cross
 

tests/docker/Makefile.include

@@ -88,7 +88,6 @@ DOCKER_PARTIAL_IMAGES += debian-s390x-cross
 DOCKER_PARTIAL_IMAGES += fedora
 endif
 
-docker-image-debian-alpha-cross: docker-image-debian10
 docker-image-debian-hppa-cross: docker-image-debian10
 docker-image-debian-m68k-cross: docker-image-debian10
 docker-image-debian-mips-cross: docker-image-debian10

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

 #
 # Docker cross-compiler target
 #
-# This docker target builds on the debian Buster base image.
+# This docker target builds on the Debian Bullseye base image.
 #
-FROM qemu/debian10
+FROM docker.io/library/debian:11-slim
 
-RUN apt update && \
-    DEBIAN_FRONTEND=noninteractive eatmydata \
-    apt install -y --no-install-recommends \
+RUN export DEBIAN_FRONTEND=noninteractive && \
+    apt-get update && \
+    apt-get install -y eatmydata && \
+    eatmydata apt-get dist-upgrade -y && \
+    eatmydata apt-get install --no-install-recommends -y \
         gcc-alpha-linux-gnu \
         libc6.1-dev-alpha-cross