Add --ignore-installed six to pip install

Fixes

    ERROR: Cannot uninstall 'six'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.

see https://open.greenhost.net/openappstack/openappstack/-/jobs/99447

Dockerfile

@@ -36,7 +36,7 @@ RUN \
   mkdir /var/cache/apk && \
   chmod a+x /usr/local/bin/* && \
   update-ca-certificates && \
-  pip install --no-cache-dir -r /requirements.txt && \
+  pip install --no-cache-dir --ignore-installed six -r /requirements.txt && \
   ln -s /usr/bin/python3 /usr/bin/python && \
   tar -xzf /tmp/flux*.tar.gz && mv ./flux /usr/local/bin && \
   npm install -g taiko@1.2.5