I tried going back to change my Dockerfile to see if that would work, and it did not:

  RUN eval $(opam env) && opam pin add dune 3.7.0 && opam install --yes dune

Similar error to before:

  [ERROR] Package dune has no known version 3.7.0 in the repositories

You're right that I could change it to grab directly from GitHub, but at that point I can also just change it to a different version of Dune, as I said above. None of this negates my original point.

What if you do "opam update" before?

I already do: https://github.com/gradbench/gradbench/blob/0276272d0df5be19...

And the CI build doesn't use Docker caching, so no shenanigans there.

Actually, I did an "opam update" (after my comment that stated there was "3.7.0") and "opam show dune --all-versions" no longer shows "3.7.0". :|

It shows:

  all-versions         1.6.3  1.9.3  1.11.4  2.3.0  2.4.0  2.5.1  2.6.1  2.7.1
                       2.9.3  3.5.0  3.6.2  3.10.0  3.12.1  3.12.2  3.15.3
                       3.17.2  3.18.2  3.19.0  3.19.1

You are right! So I suppose it is either 3.6.2 or 3.10.0. Can you use either of these versions?

Your alternative really is:

  $ opam pin add dune https://github.com/ocaml/dune/releases/download/3.7.0/dune-3.7.0.tbz

If you really must need "3.7.0".

Just FWIW, maybe it might work the way you wanted it to if you do not update the registry; worth a try. It should be able to fetch the correct tarball.