Make sure we have an up-to-date zef before installing

Alternately, maybe we should bump the version of Rakudo Star?

Author: lizmat

Dockerfile

@@ -8,6 +8,7 @@ RUN apt-get -y install zstd lrzip libssl-dev build-essential
 RUN git config --global --add safe.directory '*'
 
 COPY META6.json /srv
+RUN zef upgrade zef
 RUN zef install --force --/test HTTP::HPACK # to work around the dependency issue
 RUN zef install --force --/test --deps-only .