Add a --download-only option to pull

[samoht]

Similar to opam install --download-only that'd be great to have an option to only populate the (opam) caches.

The use-case is to repopulate caches early, so that opam monorepo pull could be run in a container with network access.

[samoht]

Just a heads-on up on this - we'll release mirage 4.0-beta next week without this feature and 4.0 probably before the end of January. Would be nice if that'd be part of the next release (I'd be happy to have a go at it if nobody else is available nor interested to implement this).

[NathanReb]

Just to clarify, in the original issue message, you meant that this should make it possible to run in a container, without network access right?

We had a recent discussion with @emillon and Raphael Proust about cache mgmt with opam-monorepo for tezos and I think we should think about using the same solutions for mirage if that's possible!

[emillon]

More precisely what we discussed was a command that reads a lockfile, and has access to the network, such that opam monorepo pull later can work without network access. Would that fit the mirage use case too?