The problem looked like this:
[root@ci1 ~]# docker image pull wzkres/centos-ci-gcc:centos-7-cross-arm64
centos-7-cross-arm64: Pulling from wzkres/centos-ci-gcc
2d473b07cdd5: Already exists
9d26d6802cad: Pull complete
90f6d29b7c1a: Pull complete
7bfff1b24796: Pull complete
80b3d4546432: Pull complete
c3a37398e8b3: Pull complete
3ba42b3a770d: Pull complete
85b3405c69c4: Downloading
2ad9c2d42a3d: Waiting
5f1b06925eb4: Waiting
4f4fb700ef54: Waiting
57c257cf6173: Waiting
787f48d693d2: Waiting
9f1677804e77: Waiting
c13fd6dc43b4: Waiting
80bd40938514: Waiting
56ba96a89fa8: Waiting
unauthorized: authentication required
When pulling public images from hub.docker.com, this error occurred randomly but very frequently, making it almost impossible to download larger images. At first I thought it was a network issue, tried different proxies and mirrors, but nothing helped. Since the error mentioned 'authentication', I suspected Docker Hub login problems and tried docker login -u xxxx, but that did not solve it.