add keep_base_url=True to support different response domain

[Lukas-Heiligenbrunner]

When gitlab responses a different base domain it is used by the gitlab lib.
In my case this was somehow wrong and setting keep_base_url=True fixed it.
With that always the env var is used.
This should fix #15
Thanks for the great project by the way.

[h44z]

thanks =)

[Voriaz]

Hello,

Just a side note on this.

I noticed that the docker image provided (with the Dockerfile) is using Python3.6 which implies using python-gitlab 2.10.
The small problem is that python-gitlab 2.10 doesn't support keep_base_url=True.
I tried to bump the docker image to Python 3.12, but I had errors with self signed certificates and it was impossible to disable cert verification globally.
I'll see if I have time to check this later and will submit a new PR.

From now, if one want to use docker image as is, he must remove keep_base_url=True from migrate.py.

Thanks,

A.