add release workflow and needed files

[ianchen0119]

Add github workflows to automatically build Free5GC docker images

[abousselmi]

LGTM, nicely done @ianchen0119

Next step would be renaming the old compose file into something like docker-compose-build.yaml and adding a new docker-compose.yaml that does not include all build steps, but uses instead pre-built images from docker hub.

[ianchen0119]

@abousselmi
Thanks for your suggestions!
I just added the docker-compose-build.yaml and updated the other necessary file.
Please help to review the changes when you feel free.

Thanks again.

[abousselmi]

Thanks @ianchen0119, I'll get to it asap