CI feature request: check maximum length of generated content against docker hub limit

[grooverdan]

25000 bytes seems to be the current docker hub limit.

[tianon]

Do you have an idea of how you think this should exhibit? We don't want a failing check (because some of them are long due to the tag listing, for example, and so having a failing check isn't super useful there). It'd be really nice if GitHub supported a trinary state for checks. 😞

(It's a little bit complicated by the fact that the limit applies to the generated contents, not the template, but we could figure out ways around that without too much trouble.)

[grooverdan]

Maybe make this additional check only apply on pull requests and not the main job.