It would be an "annoying" or "fiddly" level of work rather than a time sink costing hours and hours.
you already can do a lot by writing a bunch of scripts that will be executed on a CI, and just fiddle with differences in like metadata/artifact/how you declare your containers.
and while you are there, you can generate the code CI definitions for more than one CI provider (think of it like "multi-cloud" but for CI approach.
https://news.ycombinator.com/item?id=25340456
Continuous builds are already written in YAML + shell. Platforms like Github Actions and Travis CI basically use YAML as a really bad syntax for shell scripts (with their own if statements!), and then embed more shell as strings.
So Oil should basically generate JSON like this:
https://news.ycombinator.com/item?id=24945449
and allow you to embed unevaluated (but parsed) literal shell blocks. So I'm adding the missing "declarative" part to shell. That alone gets you pretty far.
Although there are some more language semantics that most builds need, like coarse-grained dependencies, ephemeral or persistent state, etc.
gg is a related "multi cloud" system that's very interesting:
https://lobste.rs/s/virbxa/papers_i_love_gg
I'm writing a blog post about this right now!
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An improved shell is the natural language for this problem because shell is for gluing tasks together. It's the language of heterogeneity. I view Make as a form of shell with dependencies, and Make has already been parallelized and distributed by many systems going back many years.