.gitlab-ci.yml: switch to ostc-npx-cspell for cspell

The container has all the dependencies pre-installed, making the spell
checking much faster to run.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

.gitlab-ci.yml

@@ -35,12 +35,8 @@ format:
 
 spell:
   stage: test
-  image: ubuntu:21.04
-  variables:
-    DEBIAN_FRONTEND: noninteractive
-  before_script:
-    # Install npm to get access to npx tool.
-    - apt-get update && apt-get install -y npm
+  image:
+    name: registry.ostc-eu.org/ostc/containers/ostc-npx-cspell:latest
   script:
     # Note that the particular options are duplicated with what's defined in
     # Makefile. To avoid this we'd have to get zmk as an installable dependency