How do I define a specific tag for an image in a step?

It is a good idea to use specific tags for images as it has less of a chance of being inadvertently overwritten. You can achieve this in the steps.yml step definition. If you want to use an image gitlab-registry.cern.ch/someproject:sometag you specify it via

image: gitlab-registry.cern.ch/someproject
imagetag: sometag

Why can’t you use gitlab-registry.cern.ch/someproject:sometag instead?

yadage was introduced before it was clear just how standardized these versioned strings would become. We may introduce a feature to just use a versioned string in the future.