After creating a new project via the web interface the new repository located in /home/git/repositories is owned by root:root.
This makes it impossible for the git user to push to that newly created repository.

Workaround
The current workaround is to fix that permission problems manually via

chown -R dokku:dokku /opt/gitlab

every time a new project has been created.

Ideas to fix this problem
Read this first.
Gitlab assumes that it is installed into /home/git/gitlab and the processes run as the user git.
Dynos on the other hand have /app as current working directory and the user is root (in dokku) or a random user uXXXXXX(in heroku).

Possible solutions

• make gitlab run as user git (gitlab-shell already runs as user git) or
• make gitlab create files as git user or
• change the user or the current working directory (impossible?)