While the image works, it is actually intended to be used to build other images, specially custom-made devcontainers:
# Install Oh My ZSH:
COPY --from=vovimayhem/ohmyzsh:latest \
--chown=my-username:my-username \
/home/example-user/.oh-my-zsh /home/my-username/.oh-my-zsh
# Copy recommended zshrc file:
COPY --from=vovimayhem/ohmyzsh:latest \
--chown=my-username:my-username \
/home/example-user/.zshrc /home/my-username/.zshrc
Like that, you'll have Oh My ZSH installed!