Docker overlay2 backend is broken

Due to https://github.com/moby/moby/issues/39663. It has been disabled in .gitlab-ci.yml for now.

mentioned in commit 40917e4f

mentioned in commit 6631acf9

mentioned in merge request !29 (merged)

mentioned in merge request !27 (merged)

Disabled overlay2 in !27 (merged).

mentioned in merge request !30 (merged)

closed via merge request !30 (merged)