Pin pygit2 version to 0.22.1
Pin pygit2 version to 0.22.1

Pygit2 has newer versions on pip which are incompatible with the libgit2-dev version 0.22 which is installed.  This change fixes the dockerfile so that it pins pygit2 to version 0.22.1 which is compatible.
paulwellnerbou/git-deps-docker