cleanup docker images gitlab/gitlab-runner-helper and gitlab-runner-prebuilt-x86_64?

I'm running gitlab-ci-multi-runner on a ubuntu machine for several months now. Today i cleaned up many old docker images and noticed this:

# docker images | grep gitlab
gitlab/gitlab-runner-helper               x86_64-6d06f2e      23a0614b4b27        12 days ago         50.3MB
gitlab/gitlab-runner-helper               x86_64-d24b11c      d4c4c7765499        4 weeks ago         50.3MB
gitlab/gitlab-runner-helper               x86_64-3df822b      e9635e622322        6 weeks ago         50.5MB
gitlab/gitlab-runner-helper               x86_64-f018144      5cf84c90d7b1        2 months ago        42.6MB
gitlab/gitlab-runner-helper               x86_64-adfc387      891c3ecc0633        2 months ago        42.6MB
gitlab/gitlab-runner-helper               x86_64-6104325      e955177a0be0        3 months ago        38.7MB
gitlab/gitlab-runner-helper               x86_64-0118d89      af80cfc09700        4 months ago        38.7MB
gitlab/gitlab-runner-helper               x86_64-fa8b86d      f889649fcdfe        4 months ago        38.7MB
gitlab/gitlab-runner-helper               x86_64-08a9e6f      dd563202b49b        5 months ago        38.5MB
gitlab/gitlab-runner-helper               x86_64-a67a225      510a68dddfac        5 months ago        38.8MB
gitlab/gitlab-runner-helper               x86_64-33af656      481eb92e5b03        6 months ago        38.8MB
gitlab/gitlab-runner-helper               x86_64-b32125f      c846a6aa6e74        6 months ago        38.8MB
gitlab-runner-prebuilt-x86_64             a2efdd4             3f3ba3194117        8 months ago        38MB
gitlab-runner-prebuilt-x86_64             734bc5d             31a2021fe65c        9 months ago        38MB
gitlab-runner-prebuilt-x86_64             f896af7             ab67b27f989e        10 months ago       36.8MB
gitlab-runner-prebuilt-x86_64             c66b00d             02ef8ddfa31d        10 months ago       36.8MB
gitlab-runner-prebuilt-x86_64             c52ad4f             c9ff2cbe78ca        10 months ago       39.4MB
gitlab-runner-prebuilt-x86_64             01b3ea1             9558eb1e0ec3        11 months ago       39.4MB
gitlab-runner-prebuilt-x86_64             fb49c47             bb0d1cfc227c        11 months ago       39.4MB
gitlab-runner-prebuilt-x86_64             76fdacd             b5003037ec79        11 months ago       50.9MB
gitlab-runner-prebuilt-x86_64             bcc1794             5d14d1fe3893        12 months ago       50.2MB
gitlab-runner-prebuilt-x86_64             fae8f18             19dfd0c19fa1        13 months ago       50.2MB
gitlab-runner-prebuilt-x86_64             6220bd5             1163818b4f5c        13 months ago       50.2MB

What are they?

Can they be deleted?

Why aren't they cleaned up automatically?