Can not get completions working for `g` function.

[gavvvr]

How to reproduce:

FROM ubuntu
RUN apt-get update && apt-get install -y software-properties-common
RUN add-apt-repository ppa:martin-frost/thoughtbot-rcm
RUN apt-get update && apt-get install -y vim git zsh curl rcm
RUN git clone git://github.com/thoughtbot/dotfiles.git ~/dotfiles && env RCRC=$HOME/dotfiles/rcrc rcup
RUN rcup

docker build . -t dotfiles && docker run --rm -it dotfiles zsh

If I type g and hit TAB, there is no suggestions. But I expect them to be present because there is:
compdef g=git

in zsh/completion/_g