Dockerfile: Drop world-write permission from /kas folder

The github CI environment causes the copied /kas folder to become
world-writable. This is at least undesired, so fix up the permissions.
This is done the classic way, i.e. via a separate chmod, rather than
using the new and not yet widely available "COPY --chmod".

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

Dockerfile

@@ -27,6 +27,8 @@ RUN apt-get install --no-install-recommends -y \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 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