tests/docker: move our sh4 cross compile to Buster

Now Buster is released we can stop relying on the movable feast that
is Sid for our cross-compiler for building tests.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/Makefile.include

@@ -103,9 +103,9 @@ docker-image-debian-arm64-cross: docker-image-debian10
 docker-image-debian-hppa-cross: docker-image-debian10
 docker-image-debian-m68k-cross: docker-image-debian10
 docker-image-debian-powerpc-cross: docker-image-debian10
+docker-image-debian-sh4-cross: docker-image-debian10
 docker-image-debian-sparc64-cross: docker-image-debian10
 
-docker-image-debian-sh4-cross: docker-image-debian-sid
 docker-image-debian-mips64-cross: docker-image-debian-sid
 docker-image-debian-riscv64-cross: docker-image-debian-sid
 docker-image-debian-ppc64-cross: docker-image-debian-sid

tests/docker/dockerfiles/debian-sh4-cross.docker

 #
 # Docker cross-compiler target
 #
-# This docker target builds on the debian sid base image which
-# contains cross compilers for Debian "ports" targets.
+# This docker target builds on the debian Buster base image.
 #
-FROM qemu:debian-sid
+FROM qemu:debian10
 
 RUN apt update && \
     DEBIAN_FRONTEND=noninteractive eatmydata \