Hi! There are official Docker images for snapcraft available at https://hub.docker.com/r/snapcore/snapcraft (also mentioned in the documentation) but they come with an older snapcraft version that is not able to upload the snaps to the store (I guess something has changed since then in the authentication mechanism).
The page gives instruction on how to build one’s own Docker image, but this causes an extra burden on developers – besides, those instructions look a bit hacky and I have no confidence that they’ll continue to work in the future. Instead of thousands of developers creating each one his own Docker image, it would be better if Canonical continued to provide its one.
The problem here is building and publishing snaps from the Gitlab CI: official ubuntu images come with an old version of snapcraft (3.0) which also does not support uploading snaps to the store anymore, and I cannot find a deb-based distribution that would ship a recent version of snapcraft.