Dockerfile: Test kas installation by printing its version

Same effect but less verbose and potentially confusing output.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -20,7 +20,7 @@ COPY . /kas
 RUN chmod -R o-w /kas
 
 RUN pip3 --proxy=$https_proxy install --no-deps kconfiglib && \
-    pip3 --proxy=$https_proxy install --no-deps /kas && kas --help
+    pip3 --proxy=$https_proxy install --no-deps /kas && kas --version
 
 RUN ln -s /kas/contrib/oe-git-proxy /usr/bin/
 ENV GIT_PROXY_COMMAND="oe-git-proxy" \