fab7f5c01d ci: Add missing docker.io prefix to CI_IMAGE_NAME_TAG (MarcoFalke)
Pull request description:
Currently, the CI system may pick the wrong (non-native) architecture due to the missing prefix.
For example, assuming the CI_IMAGE_NAME_TAG is `debian:bookworm` and the user has previously pulled an s390x image:
```
$ podman run --rm 'docker.io/s390x/debian:bookworm' dpkg --print-architecture
exec /usr/bin/dpkg: exec format error
```
Now, `debian:bookworm` will refer to the same image:
```
$ podman run --rm 'debian:bookworm' dpkg --print-architecture
exec /usr/bin/dpkg: exec format error
```
However, `docker.io/debian:bookworm` works fine:
```
$ podman run --rm 'docker.io/debian:bookworm' dpkg --print-architecture
arm64
ACKs for top commit:
hebasto:
ACK fab7f5c01d.
Tree-SHA512: c423c5cd454a95fa3e67081411ca08d316b8c680a5bba49196c514b91df65d9cc46a47700cc00d9579327842615f98146d0ac50abb016616a9b17d042598dab6
27101d0f0c