Move Docker repository to GitLab

[Paul Spooren]

Hi,

the repository creating all OpenWrt Docker containers (rootfs/sdk/ib) 
is currently at GitHub while all the related CI happens at GitLab(.com) 
within a mirror. As GitLab stopped their GitHub PR integration some 
time ago, PR/MRs are no longer CI tested.

I'd suggest to mirror the code to GitHub instead of mirroring it to 
GitLab, allowing PR/MR testing.

Best,
Paul