tests/docker: use debian-all-test-cross for mips

Maintaining two sets of containers for test building is silly. While
it makes sense for the QEMU cross-compile targets to have their own
fat containers built by lcitool we might as well merge the other
random debian based compilers into the same one used on gitlab.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231029145033.592566-13-alex.bennee@linaro.org>

configure

@@ -1330,6 +1330,10 @@ probe_target_compiler() {
         container_image=debian-all-test-cross
         container_cross_prefix=mips64-linux-gnuabi64-
         ;;
+      mips)
+        container_image=debian-all-test-cross
+        container_cross_prefix=mips-linux-gnu-
+        ;;
       nios2)
         container_image=debian-nios2-cross
         container_cross_prefix=nios2-linux-gnu-