Bug 1725444 - install python2 in partner-repack docker image. r=releng-reviewers,bhearsum

The partner-repack scripts use the "repo" tool from
https://github.com/mozilla/git-repo which hasn't been ported to python3
yet, so need to make sure python2 is available.

Differential Revision: https://phabricator.services.mozilla.com/D122605

taskcluster/docker/partner-repack/Dockerfile

@@ -13,6 +13,7 @@ RUN apt-get update && \
       git \
       gzip \
       openssh-client \
+      python2 \
       unzip \
       zip