CI: Network timeout connecting to registry.gitlab.haskell.org

Jobs can fail with a message like 'failed to pull image from "registry.gitlab.haskell.org..."'. The message varies slightly depending where in the runner code the exception is thrown, but it's clearly a network connectivity problem.

My first hunch was that the runners themselves are to blame. But on the first run of https://gitlab.haskell.org/ghc/ghc/-/pipelines/53940, ten jobs failed with this type of error at the same time, so maybe the problem is on the server.