automation: Add randconfig build jobs for arm64 alpine container

For arm64 we perform builds using debian and alpine containers.
We are missing the randconfig build jobs for the latter, so add them.
This way for each container we have 4 fundamental build jobs:
- defconfig non-debug/debug
- randconfig non-debug/debug

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

automation/gitlab-ci/build.yaml

@@ -568,6 +568,18 @@ alpine-3.12-gcc-debug-arm64:
   variables:
     CONTAINER: alpine:3.12-arm64v8
 
+alpine-3.12-gcc-arm64-randconfig:
+  extends: .gcc-arm64-build
+  variables:
+    CONTAINER: alpine:3.12-arm64v8
+    RANDCONFIG: y
+
+alpine-3.12-gcc-debug-arm64-randconfig:
+  extends: .gcc-arm64-build-debug
+  variables:
+    CONTAINER: alpine:3.12-arm64v8
+    RANDCONFIG: y
+
 alpine-3.12-gcc-arm64-staticmem:
   extends: .gcc-arm64-build
   variables: